55 gr Tula ammo keyholing from a M&P Sport

[murphydog]

The answer (even from nearly 7 years ago ) is probably:

Keyholing is a sign of bullet instability. This could be due to out-of-spec ammunition for that particular gun, a faulty barrel, or both. If a rifle doesn't shoot well with any ammunition of varying brands and bullet weights (as may be the case in post #17 above) it may indeed require a trip back to the factory.

 

[KidK]

Brand new M&P AR15. First 150 rounds of 55.6 Winchester target ammo without issue. Same gun, same ammo, second time to the range and two rounds failed to feed out of 60. Both had straight line indentations on the neck of the casings. Gun well oiled and clean. Maybe too wet?