Originally Posted by
Five_Point_Five_Six
A friend of mine had something similar happen. Proven barrel/upper with several thousand rounds through it, as accurate as he was able to shoot. Added a Dead Air .30 caliber can with 5.56 end cap and was getting end cap strikes. After replacing the end cap and verifying it wasn't an alignment issue, he got an end cap strike, but worse than the first.
It ended up being something funky with the barrel crown. After having the barrel recrowned, it was 100% after that.