I just completed a Mk12 assembly (Vltor MUR-1A, Centurion Arms MK12 Barrel, BCM Auto BCG, and more) and have encountered what may be a chamber that is right at the minimum spec. I shoot reloads and it appears that the rounds that I have made that fall within the case gauge specs & COAL are to tight in the chamber - ramming the shoulder into the throat causing a little resizing when chambering which in turn requires a 2 handed pull on the charging handle to eject.
I need to find a gunsmith that has actual 5.56 headspace gauges to have a look at it and figure out if I need to have the barrel sent back, or get a bolt matched to it.
I have tried 3 different BCGs (2 BCM auto and 1 PSA premium auto) and the only forgiveness I get is with my PSA BCG with over 4k rounds on it.
I know I probably need to address my sizing die, but at this point it is bottomed out on my shell plate and I can not lower it anymore to push the shoulder back any.
SO - who knows a gunsmith that knows what they are doing - and Gunsmoke is out of the question on principle.





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