Just built a second upper with a match barrel. My reloads that I always fire in my CAR-15 often will not fully go into battery on the match upper. No problems at all with factory ammo. I'll list the things I checked and see if I overlooked something.

All reloads are RCBS Full Length sized. Case trim is .010" below max. Cartridge Over All Length is OK. No burrs on case mouth. Crimp is not the issue. Sized/trimmed cases checked with a Wilson case gage.

Barrel headspace checked with GO-NO GO. Factory rounds drop right into the chamber. However, an unloaded sized/trimmed case will not fully go in the chamber. It will stick out about 1/8 of an inch, even with finger pressure.

Unless I overlooked something, I'm guessing my CAR-15 has a generous chamber and the match barrel is on the tight side and will require small base dies.

Something's not quite right..If your case gauge says it good you should be fine––you're not using 5.56 are you?

My GG&G upper needs small base, my Oly will fire anything I shove in it.

It's not a 5.56 vs .223 issue. Case gages check shoulder headspace, not case/chamber diameter.

Try giving your sizing die an 1/8th of turn down to bump the shoulder back more.



Since .223 is a rimless cartridge the shoulder stops the forward movement of the cartridge.  If you bump the should back you bump back the stopping point and should allow the bolt to lock.





I ran into the same problem with my Wylde chamber while I was using SB dies.



 

My dies are all the way down, camming over. Shoulder has been bumped back to the minimum-the rim is flush with the lower notch in the case gage. The neck diameter is a possibility. I'll smoke a couple of the reloads to see where the contact occurs.