What makes a barrel go from being able to put 10x shots 1 MOA to spreading rounds out to 4.5MOA before finishing a 30-round mag? (4x group increase happens with other ammo from baseline accuracy)
What's more is that when cool again, go back to being 1 MOA...for maybe 12 rounds and then things go back to hell...no matter the amnmo.

When everything's been checked, double checked and all you're left with is either the barrel, or maybe still its interface with the upper or muzzle device...when do you give up and call it quits?
NOTE: the upper has been rebuilt and checked for deformities in the crown, scoped for bore issue. The only observation is an above average amount of port erosion considering the round count and a comparison with 6 other barrels of similar make up and equal or greater round count.