Pressure and Brass life in a gas gun

[ReaperDriver]

Hey all, need some help from the smart folks on something I'm seeing with brass life, loose primer pockets, etc.

So quick back story and setting the scene. I have two 6.5 (.260Rem) rifles - one is an AI-AT with an aftermarket match barrel (Bartlein/Gradous reamed) and a JP barreled Gas-gun. I have been shooting the bolt gun for a number of years now with a load that is decidedly "warm". In the AI Bolt gun, I am shooting 140 Berger Hybrids in the high 2800's and low 2900s in Lapua brass and have been getting 8-12 loads easily before the primer pockets started getting loose.

OTOH, I've been struggling with the load development with the gasser and I admit I started too high based off the experience with the bolt gun and having the mindset that I should be in the roughly same speed range - obviously big mistake. I trashed most of a box of new Lapua brass during load development by trying to chase higher velocities than I should have. So I started all over from scratch and found a good lower node shooting 130 Berger hybrids in the high 2700s (2770ish) and 123 Scenars at 2850. Once I had my loads working pretty well, I shot them with a brand new box of virgin Lapua brass that I keep segregated from the bolt gun brass and neck turned, etc, etc. The JP chamber is definitely cut very tight, as I had to neck turn virgin brass just to make sure there was room for the fired case to expand. And no complaints about the JP barrel, as it's a laser beam achieving .2 to .3 MOA when I do my part.

However, I noticed that even at what should be fairly mild/"normal" loads, the primer pockets seem MUCH looser after 1 firing than the pockets of the Bolt gun running MUCH higher pressure loads. Even on the lower 130 Berger/123 Scenar loads in the gas gun - I'm getting very slight occasional swipe and I noticed when decapping them, the primers are coming out with a lot less resistance than the bolt gun primers with the same number (1x) firing on new Lapua brass. Something just feels wrong and I'm trying to diagnose what's going on. Also, I'm running JP high Pressure bolt matched and headspaced to the barrel, a JP VMOS carrier, a JP weighted Silent capture spring as well as tuning the adjustable gas port way down to just enough to function correctly. So I don't think it's either "over-gassed" or unlocking early. But maybe.....

Anyway, I'm trying to find out not only what is causing this, but also what to do about it. Do I need to go even slower velocities in the gas gun? Or just accept that gas gun brass is just not going to last the same?

 

[ReaperDriver]

For reference because someone will ask, the loads I'm running in both guns:

26" AI-AT bolt gun. 260 Rem
140 Berger Hybrid
42.8 gr of IMR 4451
Lapua Brass
Fed 210 primers
2910 fps

144 Hybrid
41.5 gr IMR 4451
All same components as above
2830 fps

22" JP gasser .260 Rem / +2 (XXL)gas length
130 AR Hybrids
41.0 IMR 4451
Lapua brass
Fed 210
2775 fps

123 Scenar
41.6 gr IMR 4451
All components same as above
2850 fps

Huge thanks in advance!

 

[FCS]

Brass from gas gun is never going to last as long as from a bolt gun.
Gas gun brass is ejected while still under pressure, bolt gun is always extracted after all pressure is gone.

I’m a fan of full mass BCG, heavier buffer and port sized specifically for burn rate and bullet weight I’ll be using.

Gas gun you can’t just be concerned about peak pressure, must also consider port pressure and gas volume.
You can have lower peak pressure but higher port pressure & more gas volume.

 

[alamo5000]

However, I noticed that even at what should be fairly mild/"normal" loads, the primer pockets seem MUCH looser after 1 firing than the pockets of the Bolt gun running MUCH higher pressure loads.

One of the F Class shooters that I have been following shoots the first loading of virgin brass at a lower pressure load to basically 'work harden' the case head.

My theory on gas gun vs bolt is that the bolt gun is much more supported on the case head throughout the firing sequence. It's not that a gas gun isn't but not 'as much' as a bolt gun.

 

[918v]

If your gas gun barrel requires neck turning then it prolly also has a short throat which in turn causes increased start pressure which then causes ejector marks and loose pp. Check that out and see if you can seat the bullet further back from the lands.