Any Thoughts On Why I Have To Bump Shoulder of New Alpha 6 ARC Brass For Gas Gun .004”?

[WeR0206]

Recently built an upper with 18” rifle +2 Ospey AR barrel in 6 ARC. It shoots great and stripped headspaced Rubber City bolt closes on a go gauge but in order for me to have easy non sticky bolt close/open I have to bump the shoulder of new alpha brass about 4-5 thou. For fired brass (in my chamber) its close to an 8 thou bump. I’m loading at 2.260” which is about 40 thou from the lands with 107 smk. Any thoughts on what’s causing the need for such a huge shoulder bump?

My FL die is a standard Redding. I have also tried an rcbs small base die but I still had to bump it about the same as redding die to get the bolt to close without sticking. My guess is the chamber was cut with a reamer at the tail end of its life so its a little tight at the base? Curious to hear the boards thoughts.

Comparative measurements:

 

[alamo5000]

Paint a piece of loaded brass with a black sharpie and chamber it. Carefully eject it and take pics.

 

[spife7980]

If the thing closes on a go gauge then the chamber isn’t “too short.”
Your numbers look alright compared to the reamer print and your relative fired vs sized. They all get smaller and are all smaller than the reamer print.

Is it dirty? A bit of build up could be enough to trick your feel.
Grab a box of factory and see if you have the same issue with hornady brass?

 

[WeR0206]

Paint a piece of loaded brass with a black sharpie and chamber it. Carefully eject it and take pics.

If the thing closes on a go gauge then the chamber isn’t “too short.”
Your numbers look alright compared to the reamer print and your relative fired vs sized. They all get smaller and are all smaller than the reamer print.
View attachment 8719256

Is it dirty? A bit of build up could be enough to trick your feel.
Grab a box of factory and see if you have the same issue with hornady brass?

Thanks guys will do. No the chamber is clean. I will def get some hornady brass/factory ammo to see how that fits.

 

[DHDeal]

I suspect it isn't the shoulder causing your issue, it's the base.

I've played with the 6.5 Grendel for a long time and the 6ARC for a couple years. I had a lot of once and twice fired 6.5 Grendel AA brass (Lapua) and it gave me headaches trying to fit that brass in a new Proof barrel. It wasn't a shoulder bump issue, it was a fat bottom. SGT of ARMS bulge buster die entered the chat and no more fat bottoms.

I don't know for sure if that is your issue, but you should not be having to bump those shoulders that far.

 

[memilanuk]

Same. I've chased that particular rabbit myself in the past, and a small base die 'fixed' the headspace issues I thought I was seeing.

 

[WeR0206]

I suspect it isn't the shoulder causing your issue, it's the base.

I've played with the 6.5 Grendel for a long time and the 6ARC for a couple years. I had a lot of once and twice fired 6.5 Grendel AA brass (Lapua) and it gave me headaches trying to fit that brass in a new Proof barrel. It wasn't a shoulder bump issue, it was a fat bottom. SGT of ARMS bulge buster die entered the chat and no more fat bottoms.

I don't know for sure if that is your issue, but you should not be having to bump those shoulders that far.

Same. I've chased that particular rabbit myself in the past, and a small base die 'fixed' the headspace issues I thought I was seeing.

Thanks for the feedback. My 6 arc build is also short stroking. The set screw bcm gas block had a super loose fit on the barrel and I didn’t do anything to bed the block. I just switched it out for a yhm clamp on block to see if that fixes the short stroking. I also measured the gas port on the 18” rifle +2 system and it seems in spec at 0.093”.

I suspect the short stroking is causing some bolt bounce or mistiming with bolt unlock causing larger headspace readings than the actual chamber size in fired brass. The size of my chamber is about 1.174 based on go gauge/my bump gauge but fired brass measures at 1.178-1.179. So im seeing a bigger difference than expected between static headspace and functional headspace.

Once I get short stroking fixed and gun tuned I will be curious to see if my shoulder measurement for fired brass goes down a few thou.

Bump gauge measurements:

• Fired brass: 1.177” - 1.179”
• 1.1901 go gauge: 1.1735” (stripped bolt closes very tight)
• New alpha brass: 1.1745” (stripped bolt won’t close)
• 1.2001 field gauge: 1.183” (stripped bolt won’t close)
• Brass at 1.1725” seems to feel as tight on bolt close as the 1.1735” steel go gauge (too tight for semiautomatic)
• To open/close bolt easily: 1.171”