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.