Grendel-like Belted Magnum Problem

[Henryrifle]

I have been shooting a 6mm ARC on and off for the past year or so. I profiled and chambered the barrel (22” Wilson w/rifle length gas), installed the extension and threaded the muzzle. The rest of the parts are reasonable quality parts and appropriate to the build. MUR Vltor upper and lower, JP bolt and Tubb flatware buffer spring. I am using a hydraulic buffer.

I’ve been shooting Hornady brass, both 6mm ARC head stamped and 6.5 Grendel head stamped that I resized. I am using a Superlative Arms AGB and have been shooting suppressed with a TBAC Ultra 9 suppressor with NO problems. I am using Hornady sizing and seating dies.

Recently I got an OSS HX QD suppressor and shot 20 or rounds of my hand loaded ammo using Hornady cases. I made no changes to the AGB and encountered no problems.

Flash forward to this weekend. I am going to be honest even though it will likely cast some doubt about judgement when it comes to reloading.

I had some virgin 6.5 Grendel brass that I reformed to 6mm ARC specifications. I measured and tested this brass before loading it to make sure it cycled normally with and without a projectile - no problem.

Here comes the judgement part - I did use a full-house load of Leverevolution with 108 ELD-Ms — not over book value but 29.4 grains. There was more neck tension than with the hornady brass but nothing outrageous - would estimate 5-7 thou of neck tension. All bullets seated to the exact depth I selected +/1 .001. Most were perfect.

Using a Labradar the MVs were an exact match for the Hornady brass with the same load — 2740 FPS. I was using a brass catcher and as the rifle was performing normally I didn’t look at the brass until I had shot 5 groups of 5.

First, I was shocked to see this - heavy swipes and ejector flow on each one:

But that was nothing compared to what I saw next:

Very lucky that the day didn't end in catastrophe!

So, what does this mean? I don't normally load full power loads in virgin brass but is that the issue here? I wouldn't think so. It seems like the bolt is unlocking while there is still significant pressure in the chamber. The new suppressor has fixed all the gas-in-the-face issues and unfired rounds in the magazine no longer have the heavy carbon dusting as when using the Ultra 9 suppressor. In fact they are as clean as they were when inserted. So, it seems like blowback is greatly reduced.

As you weigh in with your thoughts, I have shot over 1000 rounds of this same load using Hornady brass at a very similar MV. Yes there are some swipes on the Hornady brass but they are more like bright spots rather than gouges and have not caused any function issues or premature pocket enlargement. I get about 7 reloads out of the Hornady--it just happens to not be available at this time.

Lastly, I was very careful with the chamfer of the chamber. It is very lightly chamfered and the headspace is set correctly.

Help wanted....

Thank you,
Hank

 

[Evintos]

What is the ejection pattern/direction for the cases above. It could be a timing issue due to the difference in backpressure between the two different suppressors or it could be an over pressured load, could be powder lot variation, it could be temperature related since you have the same MV's but was the 2740fps taken at warmer temps and now you developed a cold temperature load shooting at the same 2740fps?

It could be a combo of the two. Leaning towards overpressure.

You are like 1 step away from case head separation or rupture.

 

[SeaBassHWD]

I have the same issues. Only my resized 6.5 Grendel Lapua brass have the same "rings" after the second firing. I'm using max CFE223 load data with a 105gr bullet. So far, they're on their 5th firing. My Hornady brass doesn't seem to do this. Curious to see what our problem is.

 

[6.5SH]

Measure the H2O case capacity of each brand of brass?

ETA:
Your gas system is too short IMO. The 18" 6mm ARC rifles are rifle +1 from the first two "official" barrel makers (which had suppressors in mind). So 1" shorter tube and 4" more length adding that much more dwell time.

 

[Henryrifle]

Update from further testing. I wonder if there is a clue in this:

I loaded 5 rounds with 1.2 less grains of Lever. So, 28.2 VS. 29.4. Everything else is was the same including outside air temperature.

Average velocity down from 2740 to 2644 FPS. Cases ejected between 4:00 and 5:00 o'clock.

Case heads look much better though a few have visible swipe marks. I single loaded the first two rounds and checked the cases after firing. Both looked normal with no sign of the belted magnum look. I loaded the remaining 3 in the magazine and shot at a normal pace--probably 30 seconds between shots as I was at the 600 yard line. All ejected between 4:00 and 5:00.

Not sure if it shows up in pictures but the first two rounds sealed in the chamber very well with little to no soot down the body of the case. The remaining 3, stripped out of the mag are increasingly sooty and you can see the belt beginning to appear a little more in shots 3, 4 & 5.

If there is a clue, I don't know what it is. Looks like it does need a little more gas. Will give it a little more next time and decrease powder again by 3/10s more to 27.9 expecting about 2625 FPS for the 108s. That will be okay. Will still be supersonic at 900 yards and retain 1000 ft-lbs of energy at 350 yards, the longest shot where I usually deer hunt.

Hopefully I'll find some more of the good Hornady brass and see if something has changed with the rifle.

Thank y'all for looking at this!

@Evintos: I am thankful that something really bad didn't happen and do know how close I was to that. It is to easy to forget that messing around with this stuff is serious business and you can really get hurt or get someone else hurt if you're not careful.

@SeaBassHWD: I have seen other people's pictures of this and really thought it was a poor chamber job. I don't believe that any longer. I too want to get to the bottom of it and hope we can identify some of the root causes in this thread.

@6.5SH: Appreciate your thoughts on gas length. Will come back to that in a second. I was a little lazy and only tested 1 case each of Hornady and Lapua.

Hornady: 34.44 grains of water
Lapua: 34.92 grains of water

Both had an equal convex meniscus.

Back to the gas system length. I am sure that running suppressed doesn't help that situation any. I imagine that an adjustable gas block doesn't change the pressure, it just reduces the volume. That could be all wrong. When your gas system is to short, can an AGB fix that by restricting the flow or is the answer: not really.

Next test, further reduce charge weight and increase gas via AGB. Will also give rifle a good cleaning and oiling!

Thank you,
Hank

 

[6.5SH]

In your case is more the length of time gas is being applied to the BCG causing it to unlock early.
The other tuning items will be buffer and spring weights to keep it closed longer.

There are way more experienced guys than me on this board regarding the intricacies of tuning a suppressed AR with excess dwell time.

 

[LRRPF52]

I’ve seen the same load put through 2 different rifles where one would kick out the belted cases and the other wouldn’t.

Both had the same gas system length.

One was 28” RLGS Grendel, the other 20” RLGS Grendel, same barrel manufacturer and chamber.

You could push the loads more with the 20” no problem, but not on the 28” RLGS.

Plug dwell time will drive your timing of the cyclic rate and extraction, and if there is too much plug dwell accumulation, you can easily move into a space where early unlocking happens.

Common configurations to be careful of this with:

18” MLGS suppressed even with a .076” port. Any gas port larger than .076” on MLGS can give you the early unlocking problems with 5.56/.223, 6mm, and 6.5 Grendel.

Throw on a suppressor and you just make it worse on most barrel lengths.

24” RLGS, 26” RLGS, and 28” RLGS also can have excessive plug dwell time.

16” CLGS is another one.

For 6mm, you have less bore volume, which means the pressure curve will be longer, especially with 103gr and heavier bullets.

It makes a lot of sense to push the gas port out farther on a 6mm to keep that port pressure down when shooting longer projectiles. The backside to that is you might run into short-stroking with lighter 6mm bullets. Same thing is true in 6.5 Grendel with 123gr vs 90gr with the longer gas systems. 90gr run great in Carbine-Length Gas for me when shooting the 12” Suppressed with TBAC Ultra 5. I had a few short-strokes with them unsuppressed.

When you change a component in the system, it’s best to do a pressure ladder work-up and see where your velocities are, if you have bolt lock on single loads with the mag, and if you’re experiencing any early unlocking symptoms as you were.

Case volume can be a bit different between Hornady and Lapua brass, but wasn’t a huge difference when I measured my own lots. But then I noticed different cyclic rate behavior between the two when trying to use the same load/bullet/COL combos.

I also am using Bootleg Adjustable Gas carriers with the 4-way choke selector through the ejection port with a small flat head. I have been using them for a few years, so I’ll update once I reach a certain round count on 2 different AR-15s in 6.5 Grendel.

 

[Evintos]

To clarify, the brass is ejecting towards you (shooter 6 o'clock) rather than ejecting forward towards the muzzle (muzzle = 12 o'clock)? If so, odd... There's a lot of signs of overpressure plus early unlock but the ejection pattern and 4-5 o'clock indicates undergassing rather than overgassing.

I don't know how temperature sensitive Leverevolution is but 30 seconds in between shots may be enough to cause a temp difference while the round is sitting in the chamber for the 30s. This could potentially explain the first two shots being ok-ish but shots 3-4 looking increasingly higher pressured. You would also see an ejection pattern shift though as well as potentially an increase in velocity if that was the case.

AGB does reduce the flow rate and can fix a short gas system issue up to a certain extent. When you get into a situation where you run out of adjustment and can't reduce gas any further, incorporating buffer or spring weight changes is required.

 

[Henryrifle]

If I am understanding y’all, the unlock time, early or late, is established primarily by the gas system length relative to barrel length (really how muck longer the bullet will be on the barrel after passing the gas port) but the ejection pattern is driven more by pressure and volume of gas entering the gas key? Guessing both are a mix so, kinda-sorta?

 

[TonyTheTiger]

If I am understanding y’all, the unlock time, early or late, is established primarily by the gas system length relative to barrel length (really how muck longer the bullet will be on the barrel after passing the gas port)

It goes both ways. Distance from chamber to the port is as much or more of a contributing factor as distance from port to muzzle.

 

[Evintos]

Unlocking timing is a combo of gas port pressure, dwell time, gas flow volume, overall system mass, gas efficiency, cam pin travel path/timing, ammunition pressure curve.

There is minimum volume of gas necessary to initial unlock. If there isn't enough dwell time, the gas vents out into the atmosphere before the BCG receives enough volume of gas. Too much dwell time and you have a excessive gas volume issue. The amount of gas flowing into the BCG exceeds the minimum required to start unlocking and it does so before excess gas vents out of the bore. The amount of volume of gas necessary is determined by how efficient your gas system is (leaky BCG? worn gas rings?) as well as the initial starting mass (semi-auto BCG, full auto BCG, low mass BCG, buffer weight), and necessary energy to compress a spring (the buffer spring plays more of a role in returning the mass back into battery).

If the gas port pressure is too high, the minimum gas volume reaches the BCG too fast starting the unlock cycle too early as well as introducing higher pressure gas that increases strain on various parts such as the bolt. If port pressures are too low, the gas doesn't flow back to the BCG faster than the bullet leaves the barrel leading to no unlock or partial unlock.

Position of the gas port relative to the chamber (pressure), gas port position relative to the muzzle (dwell), gas port diameter (constriction) determine the overall flow rate and pressure. All 3 have to be balanced properly according to the ammunition pressure curve in order to achieve proper timing.

Cam pin travel path is somewhat standardized (although there are a couple of BCGs with extended cam paths) but this dictates bolt rotational unlock speed. As the bolt carrier moves backwards, the cam pin moves along its path which allows the bolt to rotate to unlock.

Ejection pattern is determined by proper extractor tension, ejector rebound and BCG position and movement speed. BCG movement speed and position is determined by how fast the gas flow reaches the BCG and at what intensity (pressure).

If the BCG is cycling too fast, the brass in the chamber doesn't contract far enough to extract out of the chamber and when it returns forward it can attempt to load the next round in the magazine causing a double feed. If the BCG is traveling fast enough but in the overgassed condition, the brass ejects with more force into the brass deflector and this causes it to bounce forward resulting in a 1-2 o'clock ejection pattern. You can also experience even more issues on really overgassed situations such as bolt over base which is brass ejects, bounces off brass deflector but the the BCG returns forward faster than the magazine can push the next round upwards to load.

If the BCG is cycling too slow, it doesn't have enough energy to pull the brass back far enough to clear the ejection port and the brass gets kicked off the extractor claw back into the upper receiver which can cause issues like stovepipes or failures to extract. If the BCG is moving slow enough but far enough to clear the ejection port, the brass bounces off the brass deflector "softly" and this leads to a 3-4 o'clock ejection pattern.

It's a balance.

(I'll probably have to go back and edit something, too tired to proofread.)

 

[6.5SH]

There is one more to add to your well stated post. The brass deflector contact can also be from excess ejector spring strength relative to the case length. This is a well known issue with Grendel length cases and the proper fix for that is to trim a coil or two. Use of Velcro is treating the symptom not fixing the issue.

All of the timing revolves around the AR as designed. Changing anything in the system is going to require adjusting something else.

 

[LRRPF52]

If I am understanding y’all, the unlock time, early or late, is established primarily by the gas system length relative to barrel length (really how muck longer the bullet will be on the barrel after passing the gas port) but the ejection pattern is driven more by pressure and volume of gas entering the gas key? Guessing both are a mix so, kinda-sorta?

Cyclic rate is determined by multiple factors:

Propellant mass (how much powder)
Burn rate
Bore volume
Gas port location
Gas port diameter
Gas block fitment to the journal
Gas tube fitment to the block
Carrier key ID relative to the gas tube flange
Internal carrier bore dimensions relative to the bolt

Projectile mass
Projectile bearing surface engagement with the rifling
Plug dwell time (the amount of time the projectile spends forward of the gas port in a pressure-accumulated state, which can and does include suppressors)

Reciprocating mass (BCG, buffer)
Action spring weight
Coefficient of friction on any articulating surfaces (new BCG rails with rough textured rails are draggier than worn smooth rails, thick lubrication sustains longer and more consistent low friction)
Tightness of the upper receiver carrier raceway relative to the Outer Diameter of the carrier rails (some companies have made tight raceways and/or oversized carrier rail ODs)
Chamber surface texture relative to extraction (smooth allows faster extraction, rough holds onto the case longer)

Ejection pattern isn’t a reliable method for diagnosing cyclic rate but can be an indicator if all the other components in the action are correctly-made (rare).

Extractor lip corner edges and angles on the spent brass deflector can change the ejection pattern on a system that is otherwise properly-gassed.

The more you look at it, the more you start to realize how important following a well-vetted Technical Data Package is, based on extensive pyramid testing if possible.

 

[LRRPF52]

Unlocking timing is a combo of gas port pressure, dwell time, gas flow volume, overall system mass, gas efficiency, cam pin travel path/timing, ammunition pressure curve.

There is minimum volume of gas necessary to initial unlock. If there isn't enough dwell time, the gas vents out into the atmosphere before the BCG receives enough volume of gas. Too much dwell time and you have a excessive gas volume issue. The amount of gas flowing into the BCG exceeds the minimum required to start unlocking and it does so before excess gas vents out of the bore. The amount of volume of gas necessary is determined by how efficient your gas system is (leaky BCG? worn gas rings?) as well as the initial starting mass (semi-auto BCG, full auto BCG, low mass BCG, buffer weight), and necessary energy to compress a spring (the buffer spring plays more of a role in returning the mass back into battery).

If the gas port pressure is too high, the minimum gas volume reaches the BCG too fast starting the unlock cycle too early as well as introducing higher pressure gas that increases strain on various parts such as the bolt. If port pressures are too low, the gas doesn't flow back to the BCG faster than the bullet leaves the barrel leading to no unlock or partial unlock.

Position of the gas port relative to the chamber (pressure), gas port position relative to the muzzle (dwell), gas port diameter (constriction) determine the overall flow rate and pressure. All 3 have to be balanced properly according to the ammunition pressure curve in order to achieve proper timing.

Cam pin travel path is somewhat standardized (although there are a couple of BCGs with extended cam paths) but this dictates bolt rotational unlock speed. As the bolt carrier moves backwards, the cam pin moves along its path which allows the bolt to rotate to unlock.

Ejection pattern is determined by proper extractor tension, ejector rebound and BCG position and movement speed. BCG movement speed and position is determined by how fast the gas flow reaches the BCG and at what intensity (pressure).

If the BCG is cycling too fast, the brass in the chamber doesn't contract far enough to extract out of the chamber and when it returns forward it can attempt to load the next round in the magazine causing a double feed. If the BCG is traveling fast enough but in the overgassed condition, the brass ejects with more force into the brass deflector and this causes it to bounce forward resulting in a 1-2 o'clock ejection pattern. You can also experience even more issues on really overgassed situations such as bolt over base which is brass ejects, bounces off brass deflector but the the BCG returns forward faster than the magazine can push the next round upwards to load.

If the BCG is cycling too slow, it doesn't have enough energy to pull the brass back far enough to clear the ejection port and the brass gets kicked off the extractor claw back into the upper receiver which can cause issues like stovepipes or failures to extract. If the BCG is moving slow enough but far enough to clear the ejection port, the brass bounces off the brass deflector "softly" and this leads to a 3-4 o'clock ejection pattern.

It's a balance.

(I'll probably have to go back and edit something, too tired to proofread.)

I didn’t even read before I posted basically the same response. You covered it very well.

 

[Henryrifle]

@Evintos: Thank you for expanding my understanding by at least 100000%. Will read through that a few more times to let it sink in. Thanks also to @LRRPF52 for a similar explanation.

It sounds like the unlock start time is driven by gas port location and the unlock speed is driven by pressure/volume.

If that is correct, then It sounds like I have two options to fix my early unlock. I can plug the existing gas port and move it forward to or I could shorten the barrel. Is that right?

What is the right way to plug or seal a gas port?

Thank y'all!

Hank

 

[Evintos]

What is the right way to plug or seal a gas port?

Thank y'all!

Hank

Cheap solution is to get an old gas block, flip it upside down to cover it.

The other two options are welding which can potentially warp the barrel or tap and plug.

 

[Henryrifle]

Am I barking up the wrong tree or worth a try?

Hank

 

[hlee]

Am I barking up the wrong tree or worth a try?

Hank

How much is your time worth?
What is the wait time on a new barrel?
Have you exhausted other options? (Adjustable gas block, buffer weight, spring “weight,” etc.)

 

[Evintos]

I'm not a velocity chaser so if it was me, I'd just go with a lower charge weight. Make AGB and buffer/spring changes and as a last resort, chop a couple inches off the front end to reduce dwell time.

 

[Henryrifle]

I am not really a velocity chaser either but hate to run the projectile 200 FPS slow just because my rifle is set up wrong. Also, today destroying brass puts you at risk of not being able to replace it. This whole project for me has been just that, a project. There is still dykem blue just behind the gas port shoulder.

 

[Evintos]

Hm.. then lets go back to the AGB. What kind of adjustable gas block do you have and how have you adjusted it?

 

[r.tenorio671]

....I run an 18" gasser w/RLGS and SLR AGB using standard M16 BCG, standard MILSPEC buffer/tube & spring. My max test loads of 108 ELDM, Berger 105 VLD Tgt and 105 RDF's w/29.1 LVR & CCI #400's didn't give me any pressure signs.

...Pics of the brass & ejection pattern. Brass is virgin Starline 6.5G that was annealed then resized to 6mmARC. No differences noted with the brass that was not annealed before resizing, just a little more force on press handle. You can zoom in on the ejection pattern with the red box to see the ejected casings. This happens with ALL the loads/bullets I tested.

YMMV

 

[Henryrifle]

@Evintos: The SA AGB is the kind that bleeds off pressure and, if you can gauge over/under gassed by ejection pattern mine is throwing brass out between 4:30 and 5:30. I was shooting from prone and the brass was within arms reach so, 2' or so away.

I have not changed the adjustment from when I ran the Ultra 9 suppressor. Originally I adjusted it so that it would hold open on an empty mag and consistently strip rounds out of my Elander mags. No problem with either of those operations at this time.

I could try a heavier buffer but have a lathe here at home. Unless you have a more interesting plan, I am going to make a bushing to go over the existing gas port, drill a new on at Rifle +1 and retest. Before I go Rifle +2, I'll cut my barrel down from 21.75" to an inch or so shorter.

Sounds good?

Hank

P.S. @r.tenorio671: I have shot over 1000 rounds in mine using Hornady 6.5G and 6mm ARC brass with no issues either--zero. It is this new Lapua brass that has surfaced this issue. I am guessing the issue has always been there just covered up by tougher brass. As I indicated before this presents a great learning opportunity to see what changes when I change configurations.

 

[Evintos]

Potential test you can do with the Superlative Arms gas block is run it in restrictive mode with the gas completely cut off (essentially mimicking a bolt action). Test the 28.2gr or the 27.9gr loads and see if you still see pressure signs. Taking the cycling of the action as much out of the equation as possible will help determine if it's a timing issue versus something else (such as the brass).

If there is no pressure signs with those loads, run it in restrictive mode to see if the pressure sign presents itself as it would with the bleed off mode. Edit - you can also work your way up to 29.4gr again before incorporating action cycling into the equation again.

If necessary proceed with your plan of heavier buffer first then barrel gas port +1 and length shortening last.

 

[Henryrifle]

I messed around with a heavier buffer before when trying to suppress a LMT 308. That along with an adjustable gas key made it work but the forward momentum of the heavier buffer was very noticeable and I'll admit that I am much better bolt gun driver than AR driver.

All that said, I like your plan. Makes a lot of sense and is a good progression from easy to aggressive.

Will check back in with findings.

Thank you again,
Hank

 

[Dino11]

Section the brass length wise of each brand you use. I bet you will find that there is not as much meat on the bone above the case head on the Lapua brass.

 

[Henryrifle]

@Dino11: How do you interpret that? Given the popularity of the 6.5 Grendel in the AR platform, surely Lapua made their brass with semi-autos in mind? I'd be tempted to think their brass is made for bolt guns but that doesn't really make sense in this use case.

Hank

 

[EMorr]

I have a question and a wild ass guess to offer.

How much are you bumping the shoulder back on your fired brass? Personally it does not look like your primers and flattened, which make me question the "pressure signs" of the brass flowing into the ejector hole. I'm no expert, but it would seem to me that if the shoulder is bumped too far back when the firing pin strikes the primer it will push the entire cartridge into the chamber and then the expanding gas will slam the brass back into the bolt face at an extreme velocity. The further the distance the brass travels the greater the velocity the brass will strike the bolt face with, thus causing more pressure on the case head and more brass flow (theoretically, maybe), so the brass flow may be a false pressure sign.

As far as the "belted magnum" appearance of the cases, I suppose the excessive shoulder bump / headspace issue could also create a scenario where the brass expands near the base of the cartridge as the case comes to a rapid halt when it slams into the bolt face. I'd also be curious what your chamber looks like, as the "belt" looks pretty uniform on the cases you showed.

Anyway, I hope you get this solved soon and curious to hear the final solution.

edit: Can you throw a caliper on the "belt" of those fire cases and tell us how much they have expanded over a non fired case? Upon thinking more about it, it looks like the "belt" is in the area of the case that is mostly solid brass except for the flash hole and would require a metric fuckload of pressure to expand like that assuming the case is properly supported.

 

[Henryrifle]

Glad I don't have one of those screwed up barrels!

Hank

 

[r.tenorio671]

....IIRC, there was a post of sectioned 6.5G brass somewhere that showed the difference between Lapua and Hornady (?) brass. The difference was in the transitions from case head to case wall, the Lapua was "L" shaped, the Hornady (?) was "U" shaped. It was theorized that the "U" shaped transition provided greater strength in that area.

 

[Dino11]

@Dino11: How do you interpret that? Given the popularity of the 6.5 Grendel in the AR platform, surely Lapua made their brass with semi-autos in mind? I'd be tempted to think their brass is made for bolt guns but that doesn't really make sense in this use case.

Hank

I say this because all indicators point to the brass. The brass is the only thing different between his loads that work... and the ones that are giving him problems. And I have seen different brands of brass sectioned of the same caliber and have seen the difference between them. This is why we get different case capacities between different brands of brass. I speak from experience, and am only offering a different point of view. Nothing else he is doing seems to be working, aside from lowering the load substantially.

 

[Dino11]

....IIRC, there was a post of sectioned 6.5G brass somewhere that showed the difference between Lapua and Hornady (?) brass. The difference was in the transitions from case head to case wall, the Lapua was "L" shaped, the Hornady (?) was "U" shaped. It was theorized that the "U" shaped transition provided greater strength in that area.

^^^THIS^^^

 

[Laseredge]

Years ago I went through this with Alexander Arms brass (made by Lapua). It is the brass.

 

[bill alexander]

Brass will move under a residual pressure of only 1600 psi. For a fully plastic movement it will obviously depend upon the composition of the brass but I would use a pressure offset between 5400 psi and 7000 psi while running pressure/velocity measurements. This was derived by static hydraulic measurement.

With that said you can now look at your rifle set up. The Grendel, (I have no experience of the 6ARC) we could time a gun to drop to the correct residual bore pressure with rifle length gas systems up to 24" barrels. This did however demand that the gas balance was rather finely set. Carrier had to be full weight and a minimum of an H2 buffer (with the tungsten nickel iron weights not the tungsten carbide weights). Hydraulic buffers created general mayhem and only the 308 type was suitable with shorter barrels or longer gas systems, if not both. I would expect from the expansion ratio that the 6mm would be more sensitive.

Statically, the breach has to be machined correctly. The case must remain supported to a point behind the start of the case web, seen as where the case walls begin to thicken into the case head proper. This was achieved by reducing the 120 feed cone diameter on the barrel which was in turn made possible as the round would lift higher on the feed ramps. The projectile diameter placed the centerline higher when it sat on the feed ramps. Again not sure how this would look with a 6mm. Note that the support must no coincide with the case wall to web transition or a shear line is created.

If your gun has the correct dimensional set up then my immediate suspicion would be residual bore pressure from the buffer allowing the breach to open too early. Hope this helps.

 

[bill alexander]

My apologies, I note also now that you have a flat wire spring. With the spring rate of this type of spring the initial preload will be too low for the gun to cycle fully or the gun has to have a large gas port to supply enough energy to move the reciprocating parts correctly. This will have the effect of decreasing the dwell time of the action.

I was not aware that the hydraulic type buffer would fit a flat wire spring.

 

[Henryrifle]

Thank you, Bill. Hugely (Bigly) helpful! Thank you for reading all the detail and incorporating into your responses. I am learning a lot in this thread and very much appreciate the knowledge shared!

It sounds like I am doing a lot of things wrong that potentially compromise the optimum timing window for proper bolt unlock timing like, running suppressed, using a hydraulic buffer and flatwire spring, and I should probably confess to using a VLTOR buffer tube that is a little longer than mil spec.

It's no shocker that you can't just throw a bunch of cool-sounding components together and end up with a perfectly functioning system. Many choices drive compromise and you have to understand each and how they interact from a system perspective -- I don't but I'm trying!

Thank you all again for a masterclass in AR Operational Theory.

Hank

 

[Ravenworks]

Thank you, Bill. Hugely (Bigly) helpful! Thank you for reading all the detail and incorporating into your responses. I am learning a lot in this thread and very much appreciate the knowledge shared!

It sounds like I am doing a lot of things wrong that potentially compromise the optimum timing window for proper bolt unlock timing like, running suppressed, using a hydraulic buffer and flatwire spring, and I should probably confess to using a VLTOR buffer tube that is a little longer than mil spec.

It's no shocker that you can't just throw a bunch of cool-sounding components together and end up with a perfectly functioning system. Many choices drive compromise and you have to understand each and how they interact from a system perspective -- I don't but I'm trying!

Thank you all again for a masterclass in AR Operational Theory.

Hank

Pretty impressive that Father Grendel answered.

 

[supercorndogs]

I was having lots of troubles with my grendel, I switched from a flat wire to wolf extra power. Then i went to a regular spring and turned the gas way down. The thing runs like a champ now and is very accurate.

 

[357Max]

@Henryrifle - Your post peaked my interest. I've got 5lbs of Lever and 200 new Lapua 6.5 Grendel cases on my bench for 6 ARC now.
Waiting on bushings ordered a week ago that still haven't even shipped.

Looking carefully at pics in OP & post #5 I do not think your unlocking early.

The ejector swipes are not wide enough to account for full bolt unlock rotation. In fact the width of the swipes looks the same in the full load rounds as it does in the downloaded rounds. The only difference is the amount of brass flow. Note the primers are not flat on either of these.

Post #1

Post #5

More evidence to reinforce my opinion is from post 1 underside of rim. The upper right side appears to be the mark left by the extractor. I see no indication that the extractor was trying to forcibly pull the round from the chamber while still under pressure. You also stated all rounds ejected without issue.

Compare the width of your swipes to the swipe from my 22" 6 ARC.

That was a factory Hornady 108 ELDM @2635.
It's also inline with the swipe width on my 21" 6 ARC & both these barrels are rifle +2" gas. They are not unlocking early.

Here is a picture showing a mild case of the belted ring on the left. Both rounds fired from same gun. Factory 108 ammo from the 21" barrel. 6 of 50 had the ring.
50 from the 22" barrel had no ring.

In my case There are 2 contributing factors that caused the ring.

• The factory ammo has .012 - .015 head space. That much case stretch on initial firing causes false pressure signs such as the swipes.
• The 21" barrel chamber fish is coarse compared to the 22" barrel. So on some of the cases from the 21" barrel all the case stretch is happening at the unsupported portion of the case = ring of doom. On 22" barrel with slick chamber the case growth is consistently spread out over the length of the case. I cannot measure a head space difference between the 2 barrels.
  • There's 1 more thing, but I'm not opening that can of worms.

There are several unknowns that could cause Hank's result in post #1.
Several items below could be stacking.

1. What method are you using to determine where to set shoulders when reloading.
   • SAAMI spec? Are you using a case guage?
   • Or are you measuring a fired case and bumping back from there?
2. How far did you bump the shoulders back on the necked down Grendel cases? If you have a comparator what is the shoulder difference sized - fired case?
3. How tight is your bolt headspace?
   • This is really only relevant if your sizing to a case guage. Looser bolt headspace would add to case stretch.
4. Did you anneal the cases after necking down?
   • If not, the case neck/shoulder was significantly hardened during neck down and you said .005-.007 interference fit on those. having a press fit that high combined with hardened necks = a hell of a lot of grip on those loads.
5. How did you set neck tension?
   • Since this was a neck down did you just rely on the bushing for tension?
     • The above is relevant because it's not a straight neck down like 6.5 - 6 Creedmoor. Your pushing the shoulder back which can cause a doughnut to form = more added bullet grip = delayed release. Mandrel should be used on these.
6. What length did you trim to?
   • This is relevant because it stacks on Q 1 above. Even though the over all case length may be trimmed to within spec, added shoulder bump allows the neck to push farther in.
   • With a Thousand rounds on the barrel I'd bet there's some carbon in there?
7. What primer was used? Same as prior Hornady brass loads or something different?

I think if you had turned the gas off completely, the belt would've looked the same, just no swipes.

I strongly suspect you've got to much shoulder set back combined with to much effective neck tension. Pressure peaking sooner & not much if any velocity gain to show for it.

For a fully plastic movement it will obviously depend upon the composition of the brass but I would use a pressure offset between 5400 psi and 7000 psi

With .005-.007 press on hard necks the start pressure could have gone this high.


Lever Revolution is an interesting powder & It's unique characteristics may also be a contributing factor.

I've been trying a bunch of different scenarios in Gordon's reloading tool with different powders.
What seems to be unique to Lever is that it peaks pressure late like a slow powder, but somehow manages to achieve a complete burn in a short barrel unlike slow powders.

I think Hornady now shows 29.7 max load with 108/110 for Lever. That does not agree with Hodgdon (28.2 @ 110) or Gordon's also 28.0-28.2

Below I tweaked my Gordon 6 ARC data to match your rifle based on details you've given in this thread including rifle length gas.
I guessed coal @ 2.290.
Velocities are very close to what you reported 2727 @ 29.4gr and a peak pressure of 64,857.
If I shorten coal to 2.260 its 2744 @ 29.4. & 66,736 peak pressure.

Notice with the Lever, complete powder burn happens before the bullet even passes your rifle length gas tube. CFE223 peaks pressure 3/8" bullet travel sooner and needs 8" more barrel before complete burn.

I tried tweaking start pressure & it's not modeling that very well. P curve stays exact same shape, just changes peak P and timing. There's no way that's reality, but internal combustion is truly the least understood aspect left in this game.

I'll be able to give some 6 ARC data on lever in a week or 2.

 

[r.tenorio671]

^^^THIS^^^

.....FYI, over on the 6mmARC forum someone just posted this new image of cutaway's of Lapua, Starline & Hornady brass...

 

[Dino11]

The Lapua brass is noticeably thinner in the area around the case head. I am amazed that the Hornady brass is so beefy. I have a bunch of the Starline brass, some of it I have processed and used with no problems to date.

 

[LRRPF52]

There are guys who have shot the head stamps out of Lapua brass in 6.5 Grendel after 18-22 loads. Gas system and action parts need to be set up correctly, no matter the cartridge.

I had one piece of Lapua brass look like that at least 9 years ago. It was because I used too much Imperial sizing wax when I re-sized. I switched from Imperial to Hornady One-Shot spray-on lube and haven’t seen it since personally.

I sectioned the piece of brass and polished the cut to see if there were any indicators of pre-case head separation.

 

[zeroz]

Mostly I'm just in to have this thread as a reference because there is a shitload of good info here. Before I read post #39 I would have guessed dwell time problem and to whack your barrel. I didn't know what all was involved with going from 6.5 g to 6 ARC. All the more reason for you guys to leave our brass alone, we're having a hard enough time getting it, LOL. At any rate, Im interested to see if new 6 ARC brass solves the problem. At this point I'd try that first. Carry on.

 

[Henryrifle]

@357Max: I appreciate all the effort and detail you put into that response and I know others do and will as time goes by. It deserves a proper response from me.

I am going to answer the easier questions you have posed here but will need a week or two to answer the more involved questions as all hand loading efforts are focused on match preparation for next weekend.

1. Shoulder bump - I am using a Whidden Works shoulder gauge to bump the shoulders back .005" from fired brass. I just verified that the unfired virgin Lapua brass was .005" bumped back from Hornady fired brass. Here are the measurements: Fired but unsized Hornady brass (Have fired over 1000 rounds of this w/no belting) measure 1.1815. Virgin sized Lapua: 1.1765 (.005 bump) and fired/belted Lapua 1.1885 (.007 longer than fired Hornady).
2. How far did I bump from original Grendel case? - I can't answer that as I have no Grendel cases left to measure using my gauge.
3. Headspace - I chambered the rifle with .0015" of headspace from the Go Gauge.
4. Anneal after reforming - I did not anneal the cases after bumping them back to 6mm ARC size from their original 6.5 Grendel format but can report that neck tension was not out of the range high. I believe I commented earlier that neck tension was higher than average for the cartridge but not more than 5-7 thousands as estimated by feel/experience.
5. Neck Tension - I used the hornady sizing die with expander ball in it. The cases were lubed inside and out with Hornady One Shot. That might account for extra neck tension and relatively easy bullet seating--less friction but more neck tension.
6. Trim Length - I just measured 10 of them and they measure 1.4790 +/- .0005*. The Mitutoyo caliper I am using only measures down to .0005*. As I just switched to the new flow-through suppressor, I thouroughly cleaned the barrel and chamber. I have a few borescopes and know what to look for with regards to carbon ring, carbon and copper fouling.
7. Primer - In the Hornady brass, I started with CCI 450s but as the component shortage started, I switched to CCI 400s. With the same load the velocity actually picked up about 15 FPS. I used the CCI 400s in the Lapua brass as well.

I used Quickload in the development of my load and while it is not easy to account for neck tension changes other than by changing the start pressure, I am not showing pressures that high. In fact, I have tried to stay within the specified pressure envelope of the cartridge.

Please don't take any of my responses above as me being defensive of anything I am doing. I do want help and you and others have provided a lot of that! There is a problem and I'd like to find out what it is. When I get some time there are a few things I want to try:

1. Replace buffer tube with Milspec length tube
2. Exchange flat wire spring with appropriate spring
3. Exchange hydraulic buffer with appropriate weight and material buffer
4. Fire rifle with gas turned on and suppressor off
5. Fire rifle with gas turned off and suppressor off - though I don't plan to shoot this rifle unsuppressed
6. Fire rifle with gas turned off and suppressor on

I will put more time into this after next week.

Thank you,
Hank

 

[r.tenorio671]

Hank, came across this post by Bill Alexander, some interesting info on the chambers "cone" that may help you. Actually pages 8-11 have discussions on the "Grendel Belt" that may have some info you can use...

New 6mm Advanced Rifle Cartridge

Bimetal jackets were proven back in the day to increase wear in military .30 cal barrels. At the same time, the cost savings per round afford a new barrel long before you'll wear one out.

www.snipershide.com

 

[TonyTheTiger]

Haven't watched it, probably won't because I don't have 20 minutes to waste gaining 30 seconds of questionable information, but it seems pertinent so I'll post it here.

 

[357Max]

Haven't watched it, probably won't because I don't have 20 minutes to waste gaining 30 seconds of questionable information, but it seems pertinent so I'll post it here.

At 3:00 he measures the opening at the base of the dies. = .434 for the small base RCBS & .436 for the Hornady.

At 10:30 he shows what the 6.5 Lapua look like after resizing via Hornady 6ARC die. They have a ridge.

My die measures .4385 at the base (first .125 in), so .0025 bigger then the Hornady. It's an LE Wilson FL bushing die with a SAC .264 bushing.
You can feel the 6.5 neck first hit the top of the shoulder in the die before entering the bushing. Aside from feeling the step all 102 pc's of 6.5 Lapua resized to ARC easily & left no ridge at the bottom.

Here's what my 6.5G Lapua look like after resizing to 6 ARC.

I used Quickload in the development of my load and while it is not easy to account for neck tension changes other than by changing the start pressure, I am not showing pressures that high. In fact, I have tried to stay within the specified pressure envelope of the cartridge.

Please don't take any of my responses above as me being defensive of anything I am doing. I do want help and you and others have provided a lot of that! There is a problem and I'd like to find out what it is. When I get some time there are a few things I want to try:

I don't have quickload yet. Probably order it this week. I'm curious as to how well velocities are lining up for you with the quickload showing lower pressures?

Didn't take anything as defensive. We're either learning or dying. I'd also like to know what the root cause is so I can avoid it.

Ruling out shoulder bump and head space in your case. I'm leaning towards a combination of full power loads on virgin brass plus higher than usual neck tension. My thought is that the higher neck tension may be causing the lever to peak pressure sooner which would also spike the pressure higher. Add that to the Grendel type 2 bolt + the smallerish base Hornady die's and it seems a pretty good recipe for a belt.

The most common things I've seen blamed on the Grendel belt are:

• Grendel type 2 bolt that pulls the case slightly farther out of the chamber.
• Chamber mouth geometry i.e. having to big of a chamfer.
• Small base dies & it would seem Hornady while not labelled as such, leans more on the small base side of things.

All those things have been beat to death and I've seen no consensus formed aside from not using std 223 chamfer on the chamber mouth.

I think in reality the belt is resultant from a combination of 1 or more of the above plus higher than expected or recommended pressure.

My Odin barrel that belted 6 out of 50 factory 108 rounds was replaced by them & it has a pretty big chamfer. The Rainier barrel has a radius chamfer that is not as deep. It has not produced a hint of a belt yet @ 110 rounds in.

BTW Lever is some of the filthiest powder I've used & I don't shoot suppressed.

All my dies are Redding & LE Wilson for bushing dies and Mighty Armory for non bushing dies. With these I haven't noticed the over sizing a lot of people complain about.

Just fired 50 of the re-formed Lapua yesterday & had no brass issues with the Rainier barrel.
This was my first time reloading the caliber, used re-formed cases, & used bullets not available in Gordon's.

My top load for the 100gr TGK's was 29.0 Lever loaded 2.290 CCI #41 primers virgin 6.5 Lapua reformed to 6ARC via LE Wilson FL bushing die, SAC .264 bushing, & 21st Century .2415 mandrel. Lapua shoulders bumped .003 from factory Hornady fired in this chamber. Trimmed to 1.484
and annealed after sizing.

Gordon's predicted 2733fps @ 59765psi so it's close on speed anyway. I did have some very light ejector marks, but that's it.
Based on what I'm seeing I think 29.6 - 29.9 Lever for 100gr is doable with fire formed brass.
This was shots 75-80 on the barrel so it's likely still speeding up a little.

Min	2695​	Max	2731​
Avg	2719​	S-D	14.5​
ES	36​
Series	Shot	Speed
35​	1​	2730​	ft/s
35​	2​	2731​	ft/s
35​	3​	2722​	ft/s
35​	4​	2717​	ft/s
35​	5​	2695​	ft/s

Top load for 100gr PVA Seneca's was 29.3 and again just slight ejector marks. These bullets are long as hell so even loaded long there's still seated deeper into the case. Shot them @ 2.375 from a windowed mag. They're still jumping .120 @ that load length.

Pictured below are PVA 100gr Seneca, Sierra 100gr TGK, & 105 Berger Hyb.

Just 2 shots on the Magneto for the PVA's.

Min	2730​	Max	2737​
Avg	2733​	S-D	5​
ES	7​

29.3 Lever Seneca brass

The 100gr PVA's put together a pretty tight triangular shaped hole LOL.
7T at low 2700's it would appear, does not stabilize these long bastards.

It's worth noting the first round fired (center) was single loaded & the following 4 to the right were mag fed.
Hard to believe they grouped that tight when unstable.

 

[LRRPF52]

QL isn’t a safe or reliable tool for determining loads, (as they warn).

In 6.5 Grendel, the estimated pressures have been off as much as 9,000-20,000psi in both directions up and down from SAAMI MAP.

I wouldn’t expect any accurate information from QL for 6mm AR, 6mm ARC, or 6mm AR Turbo either.

As a confirmation check, you can take Hodgdon’s published pressure data for 6.5 Grendel, enter it into QL and compare the pressures. QL is way off from those, and Hodgdon’s uses calibrated pressure test breech equipment to follow SAAMI cartridge specifications.

QL is a great developer’s tool for someone with access to industry equipment. For the layman looking for loads, the data will be way off, even when the velocities are close if you have a chronograph.

 

[r.tenorio671]

...FWIW, I used virgin Starline 6.5G converted brass in my test for pressure using LVR (up to 1 step below MAX in Hodgdon's data). I had annealed the virgin brass before resizing to 6ARC. I use the Hornady Custom Dies (2 die set) and Hornady describes the sizer as a FL sizer. Pre-firing the bases measured 0.0438, post firing they ranged from 0.440 to 0.4415, on the max load cases, bullets used were 70gn to 108gn. No belting observed on cases after firing or when fired cases resized.

NOTE: 18" gasser, 1/7

 

[LRRPF52]

You can't use the same load in Hornady and Lapua brass. Lapua is much thicker and you're simply dealing with way too much pressure.

I may be wrong, but off the top of my head, I seem to recall Lapua brass being thinner but stronger, whereas Hornady has more material near the web. I just know that my Lapua brass ejects looking new, but I have also had better results with Hornady than several people who seem to cull them at higher rates.

That might be because I polished my expander ball to a mirror finish with the hopes of preventing and surface fracture failure nodes inside the neck.