.308 Gas Gun w/ Palma chamber

[MaximumVelocity]

Hey everyone.....I tried searching and came across a small bit of info but not exactly what I am looking for.

I have a custom built .308 AR10. I'm trying to at least find a good starting point for an accuracy load. The barrel is a 20" SS 1x11.25 twist made by Brux with a 95 Palma chamber. I seem to be having some problems finding a fast accuracy load. Almost everything we have tried off the shelf is too hot. I really wanted to run the Corbon 155/Lapua brass but I punched a primer with that load. Been working on some development with the SMK 155 (2156). Anything above 45.5 gr Varget is showing signs of pressure. I haven't tried 44gr-45gr yet but that is next. I really wanted it to be a bit faster.

Does anyone have a similar setup with a PALMA chamber? What seems to work for you?

 

[MitchAlsup]

Re: .308 Gas Gun w/ Palma chamber

Which pressure signs are you seeing?

Gas guns can give pressures signs if the burn rate of the powder is not appropriate for the gas port position in the barrel.

 

[MaximumVelocity]

Re: .308 Gas Gun w/ Palma chamber

I stop shooting if I see an ejector mark combined with a flat and cratered primer. I know the 95 Palma chamber is tight but I'm still a bit surprised I'm seeing these signs even at a mid range load. If the primer is okay and I see a slight ejector mark, I don't worry so much because I know gas guns can show signs of pressure but still be okay.

I really wanted to run the 155 SMK with about 46.5+ gr a Varget but it still seems to be hot even down to 45.5. I'm gunna try some 44, 44.5 and 45 then see if I can get that to group.

Another suggestion I've been given is that I may need to lap the barrel. The barrel itself might be a little tight since it's new.

 

[Xcount]

Re: .308 Gas Gun w/ Palma chamber

.
Varget is probably too slow for your configuration, i.e. port size and location down the barrel.
Try H4895, start low and work up.

 

[MaximumVelocity]

Re: .308 Gas Gun w/ Palma chamber

Makes sense....I'll pick up some H4895 and give that a go. Something I was going to do a while back but wanted to give varget a try first. Thanks for the input. I never even thought about the position of the gas port vs the burn rate of the powder.

 

[Xcount]

Re: .308 Gas Gun w/ Palma chamber

41.5g H4895 is a common good load in M1a/AR10. It is mild which usually means good accuracy. This is a good 168gr load so I would start there and work up but not too much further. I shoot 155's out of my m1a and use 42.5gr. good accuracy, I think velocity is around 2700.

gas port size and length from the chamber determine quite a bit of a load window. its all apart of the gas gun "experience".

 

[MaximumVelocity]

Re: .308 Gas Gun w/ Palma chamber

Update:

The H4895 is showing promising results. I picked up some 168gr SMK and mixed that with 41gr, 41.5gr and 42gr of H.

41gr and 42gr shot surprisingly well. 41.5 was a bit opened up

Also, 44gr Varget shot surprisingly well as just a test case.