I have a AA piston kit on an old mega mono upper.  Old stuff.   It has been a 556 and a 762x39 in its life and has worked perfect in those.  I decided to swap the barrel and bolt to a 6.8spc. I have a 6.8 bolt from ARP and it wont insert all the way into the AA piston carrier.  There is a small shelf just inside the opening where the ID shrinks some.  Is my 6.8 bolt just too fat for some reason?  Had no idea they came larger....it worked fine in my other bolt carriers, and of course my 556 and 7.62 bolts worked fine in the AA bc.

That's because Adams Arms carriers weren't exactly to spec.
I ran into the same issue and had to buy a specific bolt.
I have one available if you are interested.

Specific?

Yes. An Adams Arms piston specific super bolt from the company that cannot be named.

This gun has been a 556 and 762x39 in its life....never had an issue with those bolts.  I know the ARP bolt is beefed up in the area.