Stupid Question Sunday: Is there something inherent about gas guns that creates higher chamber pressure?

[ReaperDriver]

I apologize in advance if this is a stupid question, but I'm trying to understand the physics. The last two precision gas guns I've had - a GAP-10 .308 and a JP .260 seem to show pressure signs in loads that are well below loads that I shoot in match chambered Bolt guns of the same caliber. I get flattened primers and swipe well before I would in the bolt gun equivalent when using the same bullet and powder combo. Is there something about the gas gun mechanism itself that causes this? If so, how? Even though gas is being routed to the BCG to unlock the bolt and cycle the action - the bullet should already be out of the barrel by the time the bolt begins to unlock, right? So I would think any chamber pressure impulse has already occured before the gas port has any effect on the issue. Is it simply differences in chamber dimensions causing the higher pressure? The need for bullets to be seated deeper for mag length in gassers? Some combo of all the the above? I'm just trying to wrap my head around this. TIA.

 

[Huskydriver]

Port pressure and dwell time....

If your bolt starts unlocking early due to poor dwell time and gas length configuration and then you couple that with a high port pressure which 260 and 6.5cm are notorious for... you get excessive brass stretching and pressure signs with "normal" loads.

Most people who are serious about shooting 260 or 6.5cm in a gas gun run a +2" length gas system to shorten the dwell time and allow the pressure decreases a bit more before starting to cycle the bcg

 

[MK20]

Your gas gun has a higher internal case pressure when it unlocks and starts extracting. Even though the bullet has probably left the barrel there is still residual pressure that is in the process of equalizing. That is why gas guns are harder on brass.

 

[DIBBS]

@Huskydriver/others could you expound a bit on the plus 2 gas system?

I have a 6.5 that is overgassed, extremely hard on brasss, ejects at the 1 o-clock position. Several brands of factory loads, and like the OP handloads that show no issues in a bolt action

I have been contemplating options to remedy this such as a heavier buffer, adjustable gas block etc. I am not knowledgeable on the probable best fix.

It would be nice to be able to shoot occasionally with a can as well, which further compounds the problem.

 

[2aBaC̶a̶]

In that theory if you shut off the gas it wouldnt have the pressure problem.

Seating bullets farther in a case will increase pressure. If thats what you are doing different I would start there. Try shooting those in your bolt gun.

 

[ReaperDriver]

Port pressure and dwell time....

If your bolt starts unlocking early due to poor dwell time and gas length configuration and then you couple that with a high port pressure which 260 and 6.5cm are notorious for... you get excessive brass stretching and pressure signs with "normal" loads.

Most people who are serious about shooting 260 or 6.5cm in a gas gun run a +2" length gas system to shorten the dwell time and allow the pressure decreases a bit more before starting to cycle the bcg

My .260 definitely has the +2 XXL length gas system and I’m running a weighted SCS spring and VARIABLE mass BCG. It’s not anywhere near as bad as the GAP-10 .308 I had previously. But I still seem to get pressure earlier than I would in a bolt gun.

 

[ReaperDriver]

In that theory if you shut off the gas it wouldnt have the pressure problem.

Seating bullets farther in a case will increase pressure. If thats what you are doing different I would start there. Try shooting those in your bolt gun.

I did a ladder/OCW test yesterday with the gas turned off completely. No swipe but I was still seeing slightly flattened primers in what I would consider very low nodes. I’m going hiking now, but I’ll post some pics of the primers later.

 

[MK20]

@Huskydriver/others could you expound a bit on the plus 2 gas system?

I have a 6.5 that is overgassed, extremely hard on brasss, ejects at the 1 o-clock position. Several brands of factory loads, and like the OP handloads that show no issues in a bolt action

I have been contemplating options to remedy this such as a heavier buffer, adjustable gas block etc. I am not knowledgeable on the probable best fix.

It would be nice to be able to shoot occasionally with a can as well, which further compounds the problem.

As long as your loads are in spec, the problem is that your system is being energized by the gas too much for its corresponding weight/spring strength. Whenever this problem occurs, there are 2 options to fix without changing the ammo. Reduce the gas coming in, or increase the inertia/resistance of the reciprocating parts.

On a weapon like the FN FAL, which has a excellent gas regulating system, the easiest answer is to reduce the gas. The AR series, however, has no good gas regulation options that I am aware of. Yes, I know everyone has a gas plug that is their favorite, but by all reports they all carbon up. ETA, I see that you are asking about the +2 gas system. This will reduce your port pressure, bullet dwell time, and increase system dwell time before unlocking, so all these are good. You can do this and/or what follows in my post. You may need to do both.

The other option is to increase spring weight and/ or buffer weight. This is ideal as when you increase the weight of your moving parts, you increase the reliability of the system. What you want to shoot for is to have the reciprocating parts (buffer and bcg) move rearward, eject your brass, and gently bottom out in your receiver extension.

In other words, you want full rearward travel, but you want that travel to stop right at it has almost expended all its rearward energy. In a semi auto, recoil is most transmitted to the shooter when the reciprocating parts are halted in their rearward movement. They then transfer that inertia through the gun to the shooter's shoulder. So the slower you have those parts moving, the softer your recoil will be. If those parts are heavy but moving slowly the recoil will feel nicer compared to light parts moving fast assuming that the energy present in both is equal.

You want the closest thing to a constant recoil system you can get in a bolt gun while still having your moving parts move fully rearward every time. They need to move fully rearward for feeding reliability and so that if you ever have to use your gun in cold weather you have the extra energy present to still cycle the gun.

You will want to swap your buffer weight first with successively heavier buffers until you achieve this. If buffer weight alone doesn't do it, use a heavier spring. If you want to shoot suppressed, tune your gun unsuppressed and then put the can on. If it is too violent, keep an extra buffer of the next heavier weight around and swap that in when you shoot suppressed. When you test all this, bring your ammo to the range in a cooler full of ice, preferably dry ice covered in a towel with the ammo separate from, but still being chilled by the ice. This will simulate winter conditions and if your powder is temp sensitive will reduce its pressure accordingly which is important for proper gun setup.

Here is an example of the concept you are trying to come close to but not replicate. Skip to the 1:45 mark for the shooting but it is all worth watching.

 

[MK20]

I did what I posted above to an AR 15 I had with a 16 inch barrel. It was ejecting to the 1~2 o clock. I moved from a standard buffer all the way up to a H3 and now it ejects at the 3 o clock and the dot barely moves when I fire it. It is still completely reliable in all weather and I can shoot 0.16 splits with it while making accurate hits.

 

[Huskydriver]

My .260 definitely has the +2 XXL length gas system and I’m running a weighted SCS spring and VARIABLE mass BCG. It’s not anywhere near as bad as the GAP-10 .308 I had previously. But I still seem to get pressure earlier than I would in a bolt gun.

I did a ladder/OCW test yesterday with the gas turned off completely. No swipe but I was still seeing slightly flattened primers in what I would consider very low nodes. I’m going hiking now, but I’ll post some pics of the primers later.

Post your load data too.

If you shut the gas off completely, you are looking at the raw nature of that load in that barrel.

 

[NukeMMC]

I went thru this when I had an Armalite AR-10 with a Wilson 308 barrel and a GAP10 Gen2 in 260 but it had rifle-length gas. Check this thread. Both had AGBs (SLR and Superlative) and the 308 would leave ejector swipes when suppressed. The 260 would do it all the time, worse with the can. On those rifles, I swapped the buffer weights with tungsten, and that considerably improved brass life, but didn't completely cure the condition.

Basically the rifle-length gas system was my big culprit on the 260, but the BCG mass and the buffer mass were definite contributors. When I sold those 2 rifles, I built a Mega Maten MKM in 6.5CM with a JP +2 barrel. I also bought a Mega Maten MKM rifle length 308 upper from @black max. On the 6.5CM, I used the JP VMOS heavy bolt carrier group, the 308 has a std Aero bcg and both uppers use the same JP heavy SCS in a common lower. With an SLR AGB on both and the heavy SCS, I don't get any ejector swipes, even running my TBAC on the muzzles.

 

[2aBaC̶a̶]

I did a ladder/OCW test yesterday with the gas turned off completely. No swipe but I was still seeing slightly flattened primers in what I would consider very low nodes. I’m going hiking now, but I’ll post some pics of the primers later.

You wont get swipe with gas off. Do you see a round indentation from the ejector hole? That would be a swipe had the bolt rotated with the slightest amount of pressure.

The gas system can have an effect on the swipe as a mechanic of exctraction but not seeing how it can cause over pressure issues. Thats likely a chamber load thing. Fyi, swipe can be normal even on mild loads and with factory ammo. You can soften the edges of your ejector hole if its a problem.

 

[Huskydriver]

You wont get swipe with gas off. Do you see a round indentation from the ejector hole? That would be a swipe had the bolt rotated with the slightest amount of pressure.

The gas system can have an effect on the swipe as a mechanic of exctraction but not seeing how it can cause over pressure issues. Thats likely a chamber load thing. Fyi, swipe can be normal even on mild loads and with factory ammo. You can soften the edges of your ejector hole if its a problem.

The gas system can absolutely give a false indicator of pressure issues if your port pressure is high and the bolt unlocks early like I said above. The load can be completely low and still overly stretch the brass and gave flattened primers ejector flow etc etc

 

[NukeMMC]

You wont get swipe with gas off. Do you see a round indentation from the ejector hole? That would be a swipe had the bolt rotated with the slightest amount of pressure.

The gas system can have an effect on the swipe as a mechanic of exctraction but not seeing how it can cause over pressure issues. Thats likely a chamber load thing. Fyi, swipe can be normal even on mild loads and with factory ammo. You can soften the edges of your ejector hole if its a problem.

To further expound - My bolt guns with warm loads will leave ejector marks but not swipes. You get the swipes when the bolt turns while something is holding the brass to keep it from turning with the bolt. This is the residual pressure (dwell time-dependent) in the gas guns that is further exacerbated by adding a suppressor on the muzzle. You have to minimize the bolt movement during that gas dwell time, both to save your brass and for accuracy. Adding weight to the carrier and/or the buffer will retard bolt movement, momentum and all.

 

[2aBaC̶a̶]

The gas system can absolutely give a false indicator of pressure issues if your port pressure is high and the bolt unlocks early like I said above. The load can be completely low and still overly stretch the brass and gave flattened primers ejector flow etc etc

Is it safe to say that flattened primers are caused from the internal pressure of the case pushing the primer back out of the pocket and slamming it against the bolt face?

 

[ReaperDriver]

Post your load data too.

If you shut the gas off completely, you are looking at the raw nature of that load in that barrel.

OK, here are the pics as promised. Gun is a newly assembled JP 22" Supermatch .260 barrel with matching JP HP bolt, JP AGB, +2 XXL length gas system, VMOS carrier and SCS H2 weight.

This first set was with the gas system ON, unsuppressed. These are Lapua .243 cases necked up to .260, fired about 5x. Loads were a ladder of H4350 / 123 ELD-M / steps of 43.4 to 44.6. MV range 2965-3040 fps. No primer because I had already tumbled them in SS media the couple days before. But primers were also fairly flat. Not gross, but flat. When I shot some of these loads with the TBAC can on, it was even worse on swipe and flat primers. On the brass below, you can definitely see some of the ejector plunger (I assume) marks. There was some swipe as well, but the SS media seemed to have polished that off nicely.

The next pics below are with the gas turned off after the first two strings. The first one is Virgin Lapua / H4350 / 130 Berger AR Hybrid / 41-6-42.4gr / MV 2743 to 2785.

The one below is Virgin Lapua / H4350 / 123 ELD-M / steps of 43.2 to 44.0 / MV 2843-2900. Again not super flat primers.

These ^^ are Wolf LRP primers, BTW if that makes any difference.

It didn't shoot poorly either.

For comparison, I'm shooting 43.2 42.3 of H4350 behind a 140 Berger Hybrid @ 2910 fps in a Bolt gun and don't get anywhere near these sorts of pressure signs or flattened primers.

Bottom line is I don't think the gas gun indications are OVERLY excessive (yet) and I'm planning on trying to find one more node above these, as the speeds seem a lot slower than I would have imagined I'd get. But the pressure signs appearing at such low charge weights seems concerning and wondered if that the way ALL gas guns are or if something is improperly set up on mine.

 

[Huskydriver]

OK, here are the pics as promised. Gun is a newly assembled JP 22" Supermatch .260 barrel with matching JP HP bolt, JP AGB, +2 XXL length gas system, VMOS carrier and SCS H2 weight.

This first set was with the gas system ON, unsuppressed. These are Lapua .243 cases necked up to .260, fired about 5x. Loads were a ladder of H4350 / 123 ELD-M / steps of 43.4 to 44.6. MV range 2965-3040 fps. No primer because I had already tumbled them in SS media the couple days before. But primers were also fairly flat. Not gross, but flat. When I shot some of these loads with the TBAC can on, it was even worse on swipe and flat primers. On the brass below, you can definitely see some of the ejector plunger (I assume) marks. There was some swipe as well, but the SS media seemed to have polished that off nicely.

View attachment 7581352

The next pics below are with the gas turned off after the first two strings. The first one is Virgin Lapua / H4350 / 130 Berger AR Hybrid / 41-6-42.4gr / MV 2743 to 2785.

View attachment 7581362

The one below is Virgin Lapua / H4350 / 123 ELD-M / steps of 43.2 to 44.0 / MV 2843-2900. Again not super flat primers.

View attachment 7581367
View attachment 7581368View attachment 7581369

These ^^ are Wolf LRP primers, BTW if that makes any difference.

It didn't shoot poorly either.

View attachment 7581372
View attachment 7581374

For comparison, I'm shooting 43.2 of H4350 behind a 140 Berger Hybrid @ 2910 fps in a Bolt gun and don't get anywhere near these sorts of pressure signs or flattened primers.

Bottom line is I don't think the gas gun indications are OVERLY excessive (yet) and I'm planning on trying to find one more node above these, as the speeds seem a lot slower than I would have imagined I'd get. But the pressure signs appearing at such low charge weights seems concerning and wondered if that the way ALL gas guns are or if something is improperly set up on mine.

Your firing at near max load with the 123s and 130s and over max in your bolt gun with 140s...

If you have a tight bore in your gas gun, you will see pressure before hitting max loads.

When you necked up the 243 brass, was this time it's fireforming or did you do that in a previous fireforming?

 

[NukeMMC]

Center row is what bad swipes look like. I didn't lose any primers but had to use a swiss file to knock some of the burr down so I could get a decent sizing out of it. This was 1x fired brass loaded with 140 SMKs fired from the 20" GAP10 G2 and the can was an OpsInc 3rd Model.
Top-to-bottom rows:
42.6gr H4350 - avg. vel = 2685fps - no suppressor
42.4gr H4350 - avg. vel = 2688fps - suppressed (I rotated the raised ejector marks that I filed down to the 6 o'clock)
42.8gr H4350 - avg. vel = 2701fps - no suppressor

 

[iktomi]

Tagged for useful images.

My observations on the matter are consistent with @Huskydriver and @MK20.

 

[ReaperDriver]

Your firing at near max load with the 123s and 130s and over max in your bolt gun with 140s...

If you have a tight bore in your gas gun, you will see pressure before hitting max loads.

When you necked up the 243 brass, was this time it's fireforming or did you do that in a previous fireforming?

Ok, copy on the near max loads. I'm not disputing it, but curious what source data you're using for max. I find much of the published stuff like from Hogden to be very conservative. If my 123 and 130 loads are just about at Max, then I'll stop here with a couple of the promising nodes and do a final seating test and call it good. I just expected to get a bit more velocity out of them before it hit the top end.

The necked up .243 brass was previously fireformed, but in the AI bolt gun and then FL resized and shoulder bumped down to the gas gun's chamber specs. The gas gun headspace is about 3 thou under the bolt gun. I know that's not ideal to mix brass, but I didn't have any fireformed brass for the new gas gun and wanted to get some initial data points/starting loads before beating up my box of virgin lapua. The good news is the whole box of virgin Lapua is now fireformed to this new chamber and will be dedicated to only being used in this rifle.

Oh and sorry, I'm dyslexic today. Meant the Bolt gun load was 42.3 H4350. NOT 43.2.

 

[ReaperDriver]

Center row is what bad swipes look like. I didn't lose any primers but had to use a swiss file to knock some of the burr down so I could get a decent sizing out of it. This was 1x fired brass loaded with 140 SMKs fired from the 20" GAP10 G2 and the can was an OpsInc 3rd Model.
Top-to-bottom rows:
42.6gr H4350 - avg. vel = 2685fps - no suppressor
42.4gr H4350 - avg. vel = 2688fps - suppressed (I rotated the raised ejector marks that I filed down to the 6 o'clock)
42.8gr H4350 - avg. vel = 2701fps - no suppressor
View attachment 7581410

Wow, the suppressor really kicks the brass's ass! On a related note, does the suppressor change the OCW node? In otherwords, do I need to work up a suppressed load and an unsuppressed load for each rifle/bullet/powder combo? Or will the same OCW work for both, generally?

 

[ReaperDriver]

Speaking of .260 brass, there is Norma and Peterson brass in stock at Grafs right now, just FYI

260 Remington - Rifle Brass - Metallic Reloading - Graf & Sons

www.grafs.com

 

[NukeMMC]

Wow, the suppressor really kicks the brass's ass! On a related note, does the suppressor change the OCW node? In otherwords, do I need to work up a suppressed load and an unsuppressed load for each rifle/bullet/powder combo? Or will the same OCW work for both, generally?

OCW un-canned worked fine with the can. I didn't get that deep into it. I am sure there would have been a slight shift, just based on the velocity increase (consistently 10-12 fps on that rifle).

 

[NukeMMC]

Speaking of .260 brass, there is Norma and Peterson brass in stock at Grafs right now, just FYI

260 Remington - Rifle Brass - Metallic Reloading - Graf & Sons

www.grafs.com

Lapua: .260 Remington Cases, 100/Box

Lapua : 4PH6050 - Lapua 4PH6050: .260 Rem Cases 100/Box

www.milehighshooting.com

 

[ReaperDriver]

OCW un-canned worked fine with the can. I didn't get that deep into it. I am sure there would have been a slight shift, just based on the velocity increase (consistently 10-12 fps on that rifle).

So... I went out and tested that today. Shot my Unsuppressed OCW node with the can on (TBAC U9) and sure enough shot into one hole. There was a POI shift of about 1.1 mils low with the can on, windage was dead on still. Interestingly, the speed actually dropped about 10 fps with the can vs unsuppressed. I would have expected it to go the other way.

 

[Yondering]

Wow, the suppressor really kicks the brass's ass!

Well, yeah. Suppressors cause more residual pressure in the gas system, which causes faster/earlier unlocking. Even if your rifle is tuned perfectly without the suppressor, it’ll be overgassed when you add the suppressor.

You probably aren’t seeing pressure signs, you’re most likely seeing over gassed (i.e. early unlocking) signs.
Also, flat primers are a piss poor way to judge pressure. There is a ton of variation between actual pressure and primer appearance, and headspace has as much or more effect as pressure.

You haven’t mentioned headspace (in relation to your sized brass, not a go gauge) and how much you’re bumping shoulders back relative to fired brass from that chamber. If you’re bumping too much, that alone can cause all the symptoms you’ve described. You might find it necessary to size differently for the gas gun than the bolt gun if the chambers are significantly different.

 

[ReaperDriver]

Well, yeah. Suppressors cause more residual pressure in the gas system, which causes faster/earlier unlocking. Even if your rifle is tuned perfectly without the suppressor, it’ll be overgassed when you add the suppressor.

You probably aren’t seeing pressure signs, you’re most likely seeing over gassed (i.e. early unlocking) signs.
Also, flat primers are a piss poor way to judge pressure. There is a ton of variation between actual pressure and primer appearance, and headspace has as much or more effect as pressure.

You haven’t mentioned headspace (in relation to your sized brass, not a go gauge) and how much you’re bumping shoulders back relative to fired brass from that chamber. If you’re bumping too much, that alone can cause all the symptoms you’ve described. You might find it necessary to size differently for the gas gun than the bolt gun if the chambers are significantly different.

Agree with all. To answer some of your questions.....

1. the gas block is set up for unsuppressed currently. I did fully expect it to be overgassed when I put the can on, I just wanted to do a test to see if my OCW load held accuracy with the can on. It did really well, but there was a POI shift as expected. Eventually, I'll find the setting for suppressed and then know where to set the gas block depending on which configuration I shoot. I have a JP detent adjustable gas block, so it will be easy to count the clicks from one to the other. I just haven't gotten to that yet. The upper is brand new and I'm still sorting out basic load development. The plan is to shoot PRS Comps unsuppressed with only the TBAC brake and then shoot suppressed the rest of the time when just doing LR steel for fun.

2. I measured the Headspace for the fired round IN THAT chamber (gas gun) and then FL resize to bump the shoulder .002 back. I do not mix the brass between gas gun and bolt gun. So headspace currently should be good to go. I do the same for the bolt gun, just using a different bump amount to match that particular chamber. The cool thing is I'm using the Redding Full length Bushing dies and have the Redding Competition shellholder set. By just changing shell holders, I can control the shoulder bump without having to change the FL sizing die setup. I use the .008 shellholder for the Bolt gun and the .006 shellholder for the gas gun. Works like a champ.

 

[Greg Langelius *]

I think you may have the dreaded doughnut from using the 243 brass. I can't be sure, I'm giving secondhand advice, trying to be an internet gunsmith, somerthing I swore I'd never do.

Back in the 20-zeroes, I started shooting the 260 and was using 260 from 308 brass, which was just plain too much work (for my friend) to get up and running. Regular 260 brass was hard to keep in stock at the time. So I tried running necked up 243, and necked down 7mm-08. I was quickly advised on this forum not to use the 243, because the original neck/shoulder junction typically had a tendency to produce donuts. I never saw any problems with my 243-260 brass, but when it hardened up, I dumped it and went with 7-08 necked down. But if there is a doughnut, this is not helping your pressures, especially if you are seating bullet bases past the neck/shoulder junction.

Why 7-08 instead of 260? Compare prices, that's a part of my explanation. Another is availability, 260 brass usually dries up before 7-08, and it's a chambering which my local source (normally) carries.

Greg

 

[Yondering]

Agree with all. To answer some of your questions.....

1. the gas block is set up for unsuppressed currently. I did fully expect it to be overgassed when I put the can on, I just wanted to do a test to see if my OCW load held accuracy with the can on. It did really well, but there was a POI shift as expected. Eventually, I'll find the setting for suppressed and then know where to set the gas block depending on which configuration I shoot. I have a JP detent adjustable gas block, so it will be easy to count the clicks from one to the other. I just haven't gotten to that yet. The upper is brand new and I'm still sorting out basic load development. The plan is to shoot PRS Comps unsuppressed with only the TBAC brake and then shoot suppressed the rest of the time when just doing LR steel for fun.

2. I measured the Headspace for the fired round IN THAT chamber (gas gun) and then FL resize to bump the shoulder .002 back. I do not mix the brass between gas gun and bolt gun. So headspace currently should be good to go. I do the same for the bolt gun, just using a different bump amount to match that particular chamber. The cool thing is I'm using the Redding Full length Bushing dies and have the Redding Competition shellholder set. By just changing shell holders, I can control the shoulder bump without having to change the FL sizing die setup. I use the .008 shellholder for the Bolt gun and the .006 shellholder for the gas gun. Works like a champ.

Sounds like you have the headspace stuff figured out. You just hadn't mentioned it (and some guys really don't get it) so I figured it might be a possibility.

The suppressor POI change you saw is pretty standard, but always good to confirm your accuracy held up. I like to mark my turrets with the click value to change for suppressed use (i.e. 0.7 mil up S, or whatever it is for that rifle).

 

[ReaperDriver]

Sounds like you have the headspace stuff figured out. You just hadn't mentioned it (and some guys really don't get it) so I figured it might be a possibility.

The suppressor POI change you saw is pretty standard, but always good to confirm your accuracy held up. I like to mark my turrets with the click value to change for suppressed use (i.e. 0.7 mil up S, or whatever it is for that rifle).

Yep, I'm very pedantic about checking headspace and making sure I'm sizing correctly.

Good trick on the turret marking. I'll try that, thanks.