Inconsistent Pressure signs (Swipe/Ejector marks) on virgin brass with 6.5 "light-ish" loads

[ReaperDriver]

So, after much thrashing trying to get a good load for my .260 Rem gasser, I finally arrived on one. I worked up (actually worked down - long story) a good load in Lapua brass using the Berger 130 AR Hybrid, 41.0 gr of IMR 4451 @2790 fps. Had no pressure signs with the Lapua brass and under .5 MOA accuracy at 100 yds. I had a match coming up this weekend and wanted to use my Virgin Prime brass as I didn't want to use my new 1x fired Lapua and lose it. The Prime stuff is quite a bit lighter weight and the case volume is larger than the Lapua. So I didn't expect there would be any problems loading the same 41.0 grain load. If anything it should be slower, right?
I went out the day prior to the match to get some dope with the new loads in the Prime brass and sure enough the same load in that Prime brass was about 40 fps slower than the heavier Lapua. Accuracy was fine. But I noticed that some of the Prime brass had a significant amount of swipe while others had none. Also almost all of the cases get the plunger mark on the head. The picture below is all the same Virgin brass, all with the same load and shot on the same day. Some of the swipe actually have some sharp edges where it carved into the brass head.

So the question is 1) why am I getting this much swipe when it's a fairly light load?? 2) And then why do some show lost of swipe swipe while others are completely fine and not marked. And finally, 3) some of the virgin brass I just shot was reloaded for the match and probably 69% of them had very loose primer pockets. I would not have expected that with a lighter loads.

Is there something going on that I am not seeing? Or is Prime really that shitty and I can expect to toss right away??

TIA

 

[nn8734]

Would you happen to have a chamber drawing for your rifle’s barrel? Or know the neck diameter?

Agree that’s a sizable amount of brass overflow around that ejector plus loose pockets for that charge weight.

Wondering if you have a really tight neck chamber (but readily admit I can be totally off-base).

 

[TheOfficeT-Rex]

How far did the shoulder move, from virgin to fired?

 

[918v]

How much is your chamber allowing the case to expand? How much is the bullet jumping?

 

[FLIGHT762]

In the late 1990's, David Tubbs and others were using the SR-25 for L/R competition. They chambered some rifles in 260 Rem. and were running into problems like you are. One of the things they did to offset the port pressures using slower powders like 4350 was to lengthen the gas tube about 4". There was a Gunsmith that specialized in that at the time. I think his name is Scott Medesha.

Tubbs also brought out his own CWS (carrier weight system) to help slow down the bolt carrier. I have two of them for my 1993 SR-25 and a GAP 10. I don't think the CWS is available any more. Tubbs also has a flat spring buffer spring.

Tubbs finally gave up on the gas gun system due to the issues.

I've been hand loading for my 308 Win SR-25 since the 1990's. The guns beat the brass and can't be loaded up to bolt gun specs. My loads are 1-2 grains less than my bolt guns.

When my rifle was new, it was blowing primers with 168 FGMM until it broke in some. It has a very tight chamber. Fired cases in my rifle come out @ 1.631".

Someone who has been running a 260 gasser may be able to chime in. You're likely over gassed and may need a heavier buffer and / or an adjustable gas block.

Who made the gun?

 

[Cascade Hemi]

What does the brass look like without the gas? Ejector swipes and blown primers are not necessarily from excessive chamber pressure in a AR10/SR25.

 

[ReaperDriver]

Would you happen to have a chamber drawing for your rifle’s barrel? Or know the neck diameter?

Agree that’s a sizable amount of brass overflow around that ejector plus loose pockets for that charge weight.

Wondering if you have a really tight neck chamber (but readily admit I can be totally off-base).

I do not have a Chamber drawing. But it is a JP factory match barrel with matching headspaced bolt, so I would expect min dimensions. It does have a very tight neck. I have to neck turn Lapua brass to stay out of pressure. But the prime bass neck walls are significantly thinner, so the neck thickness shouldn't be much of an issue. A loaded neck measures: .289. A fired neck = .2965 to .970. For comparison, A loaded Lapua round with the necks turned measures about .935 to .940.

Neck wall thickness on the Prime = .0125. My Lapua necks AFTER turning = .0151. So the Prime necks are very thin to begin with.

 

[nn8734]

I do not have a Chamber drawing. But it is a JP factory match barrel, so I would expect min dimensions. It does have a very tight neck. I have to neck turn Lapua brass to stay out of pressure. But the prime bass neck walls are significantly thinner, so the neck thickness shouldn't be much of an issue. A loaded neck measures: .289. A fired neck = .2965 to .970. For comparison, A loaded Lapua round with the necks turned measures about .935 to .940.

Neck wall thickness on the Prime = .0125. My Lapua necks AFTER turning = .0151. So the Prime necks are very thin to begin with.

Gotcha.

Recalling an Erik Cortina video where he mentioned seeing pressure signs on his 6mm rifle’s brass similar to yours (some pieces were affected while others weren’t) after installing a new bbl and CW was real mild. Traced the cause to insufficient clearance at the neck and had the neck opened up by a couple thousands and the issues went away.

 

[ReaperDriver]

How far did the shoulder move, from virgin to fired?

This may be the culprit. The fired headspace (using a Hornady headspace gauge) is 1.6300. The virgin headspace from the factory was about 1.640. So that's a pretty big gap to fill. However, I shot a mix of Virgin brass and some 1x and 2x fired - and even the 1-2x fire formed cases are showing some swipe while others are not showing a mark at all on them.

This first pic is from the match with 2-3x Fired.

This pic below was the Virgin brass:

 

[ReaperDriver]

How much is your chamber allowing the case to expand? How much is the bullet jumping?

Expansion is addressed above. The bullet is jumping a decent distance. I'm loading them to Mag length of about 2.815. BTO = 2.200. Contact with the lands is about 2.85 something iirc. But the Berger 130 AR Hybrids should be pretty jump resistant. They are definition NOT being jammed if that's what you're getting at.

 

[ReaperDriver]

In the late 1990's, David Tubbs and others were using the SR-25 for L/R competition. They chambered some rifles in 260 Rem. and were running into problems like you are. One of the things they did to offset the port pressures using slower powders like 4350 was to lengthen the gas tube about 4". There was a Gunsmith that specialized in that at the time. I think his name is Scott Medesha.

Tubbs also brought out his own CWS (carrier weight system) to help slow down the bolt carrier. I have two of them for my 1993 SR-25 and a GAP 10. I don't think the CWS is available any more. Tubbs also has a flat spring buffer spring.

Tubbs finally gave up on the gas gun system due to the issues.

I've been hand loading for my 308 Win SR-25 since the 1990's. The guns beat the brass and can't be loaded up to bolt gun specs. My loads are 1-2 grains less than my bolt guns.

When my rifle was new, it was blowing primers with 168 FGMM until it broke in some. It has a very tight chamber. Fired cases in my rifle come out @ 1.631".

Someone who has been running a 260 gasser may be able to chime in. You're likely over gassed and may need a heavier buffer and / or an adjustable gas block.

Who made the gun?

I'm pretty sure it's not over-gassed. I'm using a JP XXL (2+) length adjustable gas tube. I've tuned the gas down to just cycle reliably with this load. In addition I am running a JP variable mass (VMOS) weighted BCG as well as a JP weighted silent capture spring (SCS). So there should be plenty of weight to keep it from unlocking early. I guess I could try to add more weight to the BCG. I think it comes with one tungsten weight and 2x stainless weights. So I could add more of the tungsten rings to the bolt to slow it down even more.

All the parts except the lower and upper receiver are JP. I assembled it. It shoots great, functions flawlessly and the accuracy is sub 1/2 MOA all day long when I do my part. My only issue is it shows pressure when there should not be any and those signs are is inconsistent even in the exact same conditions on the same day.

I don't get the same swipe in Lapua brass now that I've neck turned them. But I still do get that ejector pin flow on the heads. They are just not typically ripped off like I'm seeing with the PRIME stuff. Maybe the Prime is just really too soft.

 

[ReaperDriver]

What does the brass look like without the gas? Ejector swipes and blown primers are not necessarily from excessive chamber pressure in a AR10/SR25.

I have not tried turning the gas off with the Prime brass. Hmmmm, I might try that on my next range trip.

 

[ReaperDriver]

Gotcha.

Recalling an Erik Cortina video where he mentioned seeing pressure signs on his 6mm rifle’s brass similar to yours (some pieces were affected while others weren’t) after installing a new bbl and CW was real mild. Traced the cause to insufficient clearance at the neck and had the neck opened up by a couple thousands and the issues went away.

That's interesting. I do think the chamber neck dimension is very tight, given I have to turn my Lapua brass to not see those same issues. Once I started turning the lapua necks, those same issues went away. However, the PRIME brass is already WAAY under what even the turned lapua necks are - so I can't see that being the cause in this case.

 

[ReaperDriver]

I think the ejector flow marks on the head are what is causing me to pull my hair out. I get them on whatever brass I shoot and whether its a high or low pressure load. I assume that is NOT normal in a gas gun if everything is set up correctly.

 

[nn8734]

That's interesting. I do think the chamber neck dimension is very tight, given I have to turn my Lapua brass to not see those same issues. Once I started turning the lapua necks, those same issues went away. However, the PRIME brass is already WAAY under what even the turned lapua necks are - so I can't see that being the cause in this case.

Yep. Are you sizing to .004” or so shoulder bump? Brass is growing .010, that’s quite a lot.

Prime Brass could also be much softer than Lapua, brass on a semi is exposed to lots of hot gases. I get very few ejector/extractor swipes on LC brass but much more on my Federal brass when fired in my Mk11. Both loads are relatively mild. Ejection pattern is 2-2:30 with both loads.

Does recoil feel stout or sharp, more so that your load with Lapua brass?

 

[ReaperDriver]

Yep. Are you sizing to .004” or so shoulder bump? Brass is growing .010, that’s quite a lot.

Prime Brass could also be much softer than Lapua, brass on a semi is exposed to lots of hot gases. I get very few ejector/extractor swipes on LC brass but much more on my Federal brass when fired in my Mk11. Both loads are relatively mild. Ejection pattern is 2-2:30 with both loads.

Does recoil feel stout or sharp, more so that your load with Lapua brass?

I'm FL sizing and bumping the shoulder about .0035. Obviously I shot the virgin as is with that big headspace jump. My first gut feel was the big headspace difference from Virgin to Fireformed was the culprit. But then even some of the virgin Prime didn't have a mark on them (No swipe and no ejector pin flow) while others right before or after did. So something weird is going on.

 

[nn8734]

I'm FL sizing and bumping the shoulder about .0035. Obviously I shot the virgin as is with that big headspace jump. My first gut feel was the big headspace difference from Virgin to Fireformed was the culprit. But then even some of the virgin Prime didn't have a mark on them (No swipe and no ejector pin flow) while others right before or after did. So something weird is going on

and no noticeable difference in felt recoil vs your Lapua brass load?

What direction does the brass eject?

 

[ReaperDriver]

Yep. Are you sizing to .004” or so shoulder bump? Brass is growing .010, that’s quite a lot.

Prime Brass could also be much softer than Lapua, brass on a semi is exposed to lots of hot gases. I get very few ejector/extractor swipes on LC brass but much more on my Federal brass when fired in my Mk11. Both loads are relatively mild. Ejection pattern is 2-2:30 with both loads.

Does recoil feel stout or sharp, more so that your load with Lapua brass?

Sorry, hit reply before I was done..... to answer your other questions: The recoil feels pretty mild, but I'm running a TB brake. But it doesn't feel any less or more than the Lapua brass loads. Also my ejection pattern is about 3:30 to 4 o'clock after turning down the gas.

It'll be interesting to see how the primer pockets feel on the 1x fired Prime brass. If they are as loose as I've seen before, they are going in the garbage.

I've got a ton of LC .308 brass. I might play around with necking them down to .260 and see if they shoot better than the Prime.

 

[nn8734]

Sorry, hit reply before I was done..... to answer your other questions: The recoil feels pretty mild, but I'm running a TB brake. But it doesn't feel any less or more than the Lapua brass loads. Also my ejection pattern is about 3:30 to 4 o'clock after turning down the gas.

It'll be interesting to see how the primer pockets feel on the 1x fired Prime brass. If they are as loose as I've seen before, they are going in the garbage.

I've got a ton of LC .308 brass. I might play around with necking them down to .260 and see if they shoot better than the Prime.

I’ve seen loose primer pockets in 1-2x fired federal brass in 6.5 creed so wouldn’t be surprised if Prime is similar but admittedly I don’t have any hands on experience with the Prime stuff.

Doesn’t sound like overgassing to me (I’m assuming no stove piping or excessive bolt carrier speed).

Let us know how the the LC brass does…

 

[918v]

Look like you have blown primers in addition to the ejector marks. I would not rely on a comparator to measure AR brass. It is often fucked up on the bottom and stretched and does not give a reliable shoulder length reading. I would size the good ones incrementally until the bolt closes to see where the length is at.

Also test for bullet setback.

 

[Cascade Hemi]

If you can drop a bullet into the fired cases without resistance then you don't have a tight enough chamber to create excessive pressure.

Pull the gas tube and shoot a few rounds. If they do not have ejector swipes you will need to tune the gas system and/or the recoil buffer and spring. The AR308 in 260 is very sensitive to gas pressure, dwell time, and unlocking. If the rifle is unlocking under pressure it will manifest as excessive pressure (ejector swipes, blown primers, stretched primer pockets). Slower burning powder with exacerbate the problem because it makes pressure slower and maintains pressure longer.

ETA: Not all powders and bullets are compatible with all gas guns. 4451 is a slow powder and probably poorly suited for this application. Also, 41.0gr of IMR 4451 isn't a soft load, it is around 58K PSI and over max for the platform.

 

[ReaperDriver]

Look like you have blown primers in addition to the ejector marks. I would not rely on a comparator to measure AR brass. It is often fucked up on the bottom and stretched and does not give a reliable shoulder length reading. I would size the good ones incrementally until the bolt closes to see where the length is at.

Also test for bullet setback.

No. No blown primers. I started decapping some to throw in the tumbler before I thought to post. What spurred me to start a thread was how loose the primer felt coming out on the just fired virgin batch.

Also, no issue with bullet set back. I've tested that several times.

 

[ReaperDriver]

If you can drop a bullet into the fired cases without resistance then you don't have a tight enough chamber to create excessive pressure.

I did this first thing. No issue with the prime brass, bullet slides right in. This is how I discovered I had to neck turn my Lapua brass because a bullet would definitely NOT slide into the fired lapua case.

Pull the gas tube and shoot a few rounds. If they do not have ejector swipes you will need to tune the gas system and/or the recoil buffer and spring. The AR308 in 260 is very sensitive to gas pressure, dwell time, and unlocking. If the rifle is unlocking under pressure it will manifest as excessive pressure (ejector swipes, blown primers, stretched primer pockets). Slower burning powder with exacerbate the problem because it makes pressure slower and maintains pressure longer.

I have done this in the past with the Lapua brass but not the Prime. Turning the gas port off doesn't get the swipe, but it still got the ejector dimple.

 

[ReaperDriver]

ETA: Not all powders and bullets are compatible with all gas guns. 4451 is a slow powder and probably poorly suited for this application. Also, 41.0gr of IMR 4451 isn't a soft load, it is around 58K PSI and over max for the platform.

I didn't know that about IMR 4451. It supposedly is the same or nearly the same on the burn chart right next to H4350 and everyone uses that powder. What would you recommend instead for a faster powder for use in this application?

 

[Cascade Hemi]

I did this first thing. No issue with the prime brass, bullet slides right in. This is how I discovered I had to neck turn my Lapua brass because a bullet would definitely NOT slide into the fired lapua case.


I have done this in the past with the Lapua brass but not the Prime. Turning the gas port off doesn't get the swipe, but it still got the ejector dimple.

Primer cratering is a separate issue unrelated to unlocking under pressure. Excess pressure can cause cratering but looking at your pictures I don't think that is your issue.

 

[308sako]

If you try the LC brass I would strongly suggest to do a water test for volume comparison after the forming but before using identical charges.

 

[Cascade Hemi]

I didn't know that about IMR 4451. It supposedly is the same or nearly the same on the burn chart right next to H4350 and everyone uses that powder. What would you recommend instead for a faster powder for use in this application?

Hodgdon makes that claim but everything I see says otherwise. H4350 makes peak pressure sooner. You may need to try a few different powders and I would lean faster rather than slower. You need to try different combinations of gas, buffer weight, and recoil spring. Less weight, less gas, and more spring for example.

 

[Bullspotter]

How much case grip? Are you letting the rifle feed from the magazine every shot? Thinner necks? = less case grip? Bullets sliding out of case when bolt slamming shut. Fire one round and then extract the loaded round from the chamber, fire another and extract the next one from the chamber, do this till you have 4 or 5 that were chambered by the rifle cycling and check COAL on those 4-5 rounds.

 

[918v]

No. No blown primers. I started decapping some to throw in the tumbler before I thought to post. What spurred me to start a thread was how loose the primer felt coming out on the just fired virgin batch.

Also, no issue with bullet set back. I've tested that several times.

The blue box has cases with blown primers. The black around the circumference of he primer indicates it is blown.

 

[FLIGHT762]

I've got a ton of LC .308 brass. I might play around with necking them down to .260 and see if they shoot better than the Prime.

I have a Remington Mountain Rifle in 260 Rem. I've been reloading for a number of years. If I'm not using 260 brass, I make 260 from 243 Win. cases. I bought a tapered expander from RCBS and it does a nice job. I use tapered expanders to go from 308 to 358 and 30/06 to 338 / 06. They work.

Most recommend using 7-08 cases to neck down to 260, but i ended up getting a lot of free 243 cases and they work fine.

I've tried sizing down 308 Win. cases to 260 and the only cases that worked without having too thick necks were the old Hornady and Winchester cases that weighed under 160 Grs.

For me, LC 7.62 NATO & Federal brass has necks way too thick for my rifle.

Try your LC cases, just be aware.

 

[Skg_Mre_Lght]

I have a 6.5 gas gun that I fought pressure signs with after two barrels, different bolts, etc.

After doing research, it was evident that everyone has pressure sign issues with 6.5 gassers. I ended up ordering a Criterion barrel with the matched/headspaced JP high pressure bolt with the small firing pin, and an SLR adjustable gas block. I finally got it dialed in, and it runs flawlessly.

The prevailing opinion is that 6.5s are hard on the ejection cycle and way over gassed even in a rifle length gas system compared to a lower pressure 308. This is why you see quite a few "+2" 6.5 barrels. It's so overgassed that it strips the round from the chamber prematurely (before the pressure dies off), causing swipe and ejector marks, and the HP bolt helps because it absolutely slams the case into the bolt face.

Hope that helps.

 

[nn8734]

@ReaperDriver - next time you go out, bring a set of dial calipers or 1” micrometer with you and measure each one of your rounds before and after firing. Measure the case web just above the extractor groove and calculate the expansion. Anything more than .0005” indicates an overpressure condition. That should definitively rule in or out chamber over pressure for your new test loads. And start low with the LC cases (I’m assuming you’re using LC LR 1x fired M118LR brass).

 

[Cascade Hemi]

The only difference between a HP bolt and a standard bolt is the firing pin hole diameter. 6.5CM, 260 Rem, and 308 Win are all effectively the same SAAMI chamber pressure. The main issue with 6 and 6.5, port diameter not withstanding, is the burn rate necessary for the smaller bore; the action begins to unlock before the pressure drops low enough. Peak port pressure isn't higher but the duration is significantly longer. The more dwell time the worse it gets (barrel length in front of the gas port). AR308s are the deep end of the pool and you can have all of these issues with 308 chamberings too. Changing barrels is not a guaranteed fix, the only real way to guaranty you don't have this issue is to buy a rifle from a company like KAC and shoot only the ammo they designed the rifle for.

You can buy different barrels until you end up with one that works for your load or you can go through trial and error with what you have now. If you want to make what you have work, my advice with picky AR308s is to run the fastest powder and lightest acceptable bullet you can and do pressure ladders under gassed. Use less buffer weight, more spring, and sneak up on getting the rifle to function. Most people go straight to more buffer weight and just end up with more recoil and the same wrecked brass.

 

[ReaperDriver]

The blue box has cases with blown primers. The black around the circumference of he primer indicates it is blown.

Ok, maybe we just have different definitions of what a "blown" primer is. I always thought that it was either a pierced primer (hole through the primer strike) or the primer has been physically blown out of the case. Neither of those situations were the case with these particular loads. Although I have seen this in the past. Now, If you are talking about gas coming out of the primer pocket around the sides of the primer itself, then that's certainly possible. But all of the cases in that pic had their primers and there were no pierced primers. The ones without the primer were simply decapped before I thought to take a pic and post this thread.

 

[ReaperDriver]

I have a Remington Mountain Rifle in 260 Rem. I've been reloading for a number of years. If I'm not using 260 brass, I make 260 from 243 Win. cases. I bought a tapered expander from RCBS and it does a nice job. I use tapered expanders to go from 308 to 358 and 30/06 to 338 / 06. They work.

Most recommend using 7-08 cases to neck down to 260, but i ended up getting a lot of free 243 cases and they work fine.

I've tried sizing down 308 Win. cases to 260 and the only cases that worked without having too thick necks were the old Hornady and Winchester cases that weighed under 160 Grs.

For me, LC 7.62 NATO & Federal brass has necks way too thick for my rifle.

Try your LC cases, just be aware.

That's a good point, thanks. I will definitely check them if I get around to trying the LC stuff. Worst case, I can just turn the necks down if they end up too thick. I've got tons of .308 brass of all headstamps, so may try a couple.

Probably simplest solution is to just get a brass catcher for the matches so I don't loose my good Lapua brass. I've got several boxes of virgin Lapua, so am not short of brass. I just didn't want to lose them at over a $1 a piece now.

 

[ReaperDriver]

The only difference between a HP bolt and a standard bolt is the firing pin hole diameter. 6.5CM, 260 Rem, and 308 Win are all effectively the same SAAMI chamber pressure. The main issue with 6 and 6.5, port diameter not withstanding, is the burn rate necessary for the smaller bore; the action begins to unlock before the pressure drops low enough. Peak port pressure isn't higher but the duration is significantly longer. The more dwell time the worse it gets (barrel length in front of the gas port). AR308s are the deep end of the pool and you can have all of these issues with 308 chamberings too. Changing barrels is not a guaranteed fix, the only real way to guaranty you don't have this issue is to buy a rifle from a company like KAC and shoot only the ammo they designed the rifle for.

You can buy different barrels until you end up with one that works for your load or you can go through trial and error with what you have now. If you want to make what you have work, my advice with picky AR308s is to run the fastest powder and lightest acceptable bullet you can and do pressure ladders under gassed. Use less buffer weight, more spring, and sneak up on getting the rifle to function. Most people go straight to more buffer weight and just end up with more recoil and the same wrecked brass.

To your point about less carrier weight - that seem counterintuitive to the problem of the bolt unlocking early. I spoke with the JP guy at length when I bought the parts to build the rifle and he recommended the heavier buffer over their lighter buffer for the exact reason of keeping the bolt locked longer. I'm not saying you're wrong, but what is the physics or rationale behind behind going with a lighter buffer and heavier spring vs heavy/heavy??

 

[918v]

Ok, maybe we just have different definitions of what a "blown" primer is. I always thought that it was either a pierced primer (hole through the primer strike) or the primer has been physically blown out of the case. Neither of those situations were the case with these particular loads. Although I have seen this in the past. Now, If you are talking about gas coming out of the primer pocket around the sides of the primer itself, then that's certainly possible. But all of the cases in that pic had their primers and there were no pierced primers. The ones without the primer were simply decapped before I thought to take a pic and post this thread.

Blown primer means gas leaked around the primer pocket. This will fuck up your bolt face.

 

[Cascade Hemi]

To your point about less carrier weight - that seem counterintuitive to the problem of the bolt unlocking early. I spoke with the JP guy at length when I bought the parts to build the rifle and he recommended the heavier buffer over their lighter buffer for the exact reason of keeping the bolt locked longer. I'm not saying you're wrong, but what is the physics or rationale behind behind going with a lighter buffer and heavier spring vs heavy/heavy??

Because you're turning the gas down at the same time. The idea is to reduce the amount of energy going through the gas system and the recoil system while retaining feeding reliability. Less gas, later unlocking, less energy necessary to cycle the action, and less energy moving back and forth. I know it is counterintuitive but you're trying to effect the unlocking timing. Frankly, I'd start with a traditional buffer, a pile of tungsten weights, and Sprinco blue and red springs. Turn the gas down until the gun doesn't cycle. Then start adjusting up; a little more gas, until it cycles. Check for swipes. Add some buffer weight. Add more spring if it doesn't strip the top round off of the mag. Again, most people just add weight and I can tell you from experience that a 10oz buffer doesn't make the gun run better even if it reduces the swipes.

 

[ReaperDriver]

Because you're turning the gas down at the same time. The idea is to reduce the amount of energy going through the gas system and the recoil system while retaining feeding reliability. Less gas, later unlocking, less energy necessary to cycle the action, and less energy moving back and forth. I know it is counterintuitive but you're trying to effect the unlocking timing. Frankly, I'd start with a traditional buffer, a pile of tungsten weights, and Sprinco blue and red springs. Turn the gas down until the gun doesn't cycle. Then start adjusting up; a little more gas, until it cycles. Check for swipes. Add some buffer weight. Add more spring if it doesn't strip the top round off of the mag. Again, most people just add weight and I can tell you from experience that a 10oz buffer doesn't make the gun run better even if it reduces the swipes.

Ah... that makes sense now. Thanks.

 

[ReaperDriver]

Hi all, a buddy of mine sent me this link that is super informative about all the stuff we discussed here. It's specifically for AR-15s, but I assume the concepts should be identical to large frame ARs as well.

AR15BARRELS.COM - Randall's description of AR gas operation and how everything works in harmony

How the AR-15 gas system works

www.ar15barrels.com

It's given me a lot to consider and its helping to diagnose what's going on with my .260 rifle. Curious what you guys think of it and if there are any aspects that don't apply to the AR308 systems or are incorrect.

 

[ReaperDriver]

Ok, to close the loop on this - I'm pretty sure I found my root cause today. I was trying to go through all of the variables that could be causing these obvious pressure signs and torn up brass. It turns out (or at least I'm pretty sure) it has been a headspace issue. I had been bumping the shoulders about .0025 to .003 based on my understanding that typically a .002 shoulder bump in a bolt gun and about .003 in a gas gun is desirable for smooth function. Turns out it was too much for this particular rifle. After discussion with my shooting partner last night, I took some 3x fired cases (PRIME brass) and set up the die to just barely bump them enough to chamber and eject the live round without having to force the bolt open. So I started with a fired round with no sizing and it would chamber but would not eject without having to bang the stock against the loading table while holding the charging handle to get it open. I backed off in tiny increments until it would open normally. Turns out that sweet spot was about .0005 to .0015 bumped from the fired cases. I'm using the Hornady headspace tool and a Mitutoyo digital caliper. Fired rounds measure 1.6300-1.6305 and the final bump to get them to eject normally was 1.6290-1.6295.

So I went out and shot them today along with a batch of the same brass and same exact load, but with the brass headspaced to my original dimensions of around 1.6265-1.6370. HUGE difference. The minimal headspace brass had zero indications of swipe or the ejector stamp where the brass flowed into the ejector hole. And best of all, it functioned flawlessly even with the minimal bump. I have a small set of data points, but I'll continue to barely bump the shoulder and see if it continues to not show the swipe and ejector "nipple" shearing like I was seeing before. But I would not have believed that that small a change in headspace / shoulder bump could have made that big of a difference.

Below are cases from the original .003 headspace.

Below are cases with the minimal shoulder bump

the case in the middle of the blue box was where I tried to file off a previous ejector bump and got a little too aggressive. So that was there previously.

All the cases in the pic where shot today. Same conditions, same exact load, etc.

 

[918v]

Did you ever check how much your rifle is bumping the shoulder while chambering, if any?

 

[308sako]

While I'm not a huge fan of reading primers for pressure, it APPEARS that your tight head spaced brass showed "less" pressure as to how "flat" the primers are.

 

[ReaperDriver]

Did you ever check how much your rifle is bumping the shoulder while chambering, if any?

Yep. If I take a fired case and chamber it, it bumps maybe a half thousandth. If I size and bump to that .001 to .0015 like above, there is no additional bump from chambering.

 

[918v]

Yep. If I take a fired case and chamber it, it bumps maybe a half thousandth. If I size and bump to that .001 to .0015 like above, there is no additional bump from chambering.

Lucky… I wish mine was like that. Glad you got your problem worked out.

 

[ReaperDriver]

Lucky… I wish mine was like that. Glad you got your problem worked out.

Fingers crossed. Like I said, it was a very small data sample, so I might not be out of the woods yet. But it looks promising. And as @308sako said, the pressures signs were pretty much completely gone and the rounds shot basically into one ragged hole. Also, they were right on the expected node of 2790 with the load of 41.2gr of 4451 pushing a 130 Berger Hybrid in about 102F conditions. I use an even 41.0 in Lapua brass to get the same speed because the Prime brass has quite a bit more case capacity than the Lapua stuff. I'm going to load up 30-50 and go shoot at longer ranges and see how it does and see if the lack of pressure signs are repeatable.

 

[Cascade Hemi]

Glad it was that easy, it isn't usually.

 

[ReaperDriver]

Glad it was that easy, it isn't usually.

Thanks. Well, like I said - the jury is still sorta out. Once I can retest and duplicate it, I'll know for sure. but it looks promising.....

 

[Perchowski]

I had a similar issue with an AR-10 gas gun chambered in 6.5 creedmoor. Was getting bad ejector marks on the brass and primer pocket wouldnt hold a primer after 1-2 reloads in Norma brass.
I tracked it down to;
1) not bumping the shoulder back (I was very new to reloading at the time and didn't even know bumping the head back was a thing) so my bullets were against the lands.
2) premature bolt unlocking. Even though I was running a JP H2 SCS and their VMOS BCG in the heavier configuration, my bolt was unlocking too early and starting to eject the case before pressure dropped to an acceptable level. I had to add a tungsten weight to the SCS to make it an H3 buffer equivalent.
After I started bumping the head back ~.003 and seating the bullet .01 deeper, as well as adding the extra weight to prevent premature unlocking of the bolt, not only was I not seeing any pressure signs but I also gained ~50fps! (also added a slight crimp with the lee collet crimp die)
Load for that rifle is Norma brass, CCI #34 primers, 42.3gr H4350 behind 140gr. Barnes match burners seated to ~2.790 ±.008, pushing them @ 2800fps out of a 22in. JP barrel. It's basically a LRP-07 clone only I run their VMOS BCG since I only shoot that rifle suppressed.

Edit: before anyone asks, the reason for the .008 variance in seating depth is due to inconsistent ogive on the Barnes match burners. When I seat 140gr ELD-M there is only ~.002 variance.

 

[Perchowski]

Ah... that makes sense now. Thanks.

Fyi I have a JP lrp-07 clone I built using a juggernaut tactical upper/80%lower and had the same issues. I run their VMOS BCG instead of the LMOS because i run the rifle suppressed 100% of the time.
I had Ejector swipes on some cases from box ammo, sometimes the brass was fine with no pressure signs..
When I started reloading even lighter loads did the same thing.
I ended up writing a comment on one of their YouTube videos regarding my issue. 2-3 days later, they posted another video that directly addressed my issues and it came down to the bolt unlocking prematurely.
My brass always came out extremely dirty because the bolt was unlocking too early and breaking the seal of the case while pressure was still high, causing gas to rush passed the brass and dirty it all up.
I ended up having to add more buffer weight to the SCS buffer, making it an H3 with all tungsten weights plus I had their VMOS BCG with the heavier 2 tungsten weights on it.
After I added more weight to the buffer system, I not only picked up a little velocity but my brass came out looking normal finally and there are no more pressure signs on brass.
I hope this helps you in your situation but hard to diagnose over the interwebs.