See if the problem is a head space problem, or a leade problem.


Upper in hand with no B/C, drop a factor round into the chamber, tap the back of the round with a wooden dowel to fully seat the round against the chamber shoulders, then tilt the muzzle up to see if the factory round will fall free.   From there, break out a head space gauge, and check the head space of the bolt to barrel.

Also, check the barrel extension lugs (inside) for any burs that could be causing the barrel to head space too tight.

Lastly, pull the bolt off the carrier and with just the carrier with key, dry fit them into the upper to make sure that the key it not too wide and binding in the upper slot for it, and the gas tube is correctly aligned for the carrier key.

If the factory round will not fall out of a "chamber brush with CLP  scrubbed cleaned chamber"  when the round is seated fully in place (wooden dowel), then the barrel needs to go back.

If these are reloads, then put the reloads through an ammo test gauge first.  With reloads, the cases may not have been fully sized correctly to begin with, or too much bullet crimp pressure used to bulge the case shoulders instead.