are we helping with a 14" bbl or an 8" one here?Just pulled a range target from May when I shot my AR 5.56 pistol at 25 yards and dang if they weren't all tumbling! I blamed it at the time on the ammo which was milsurp frangible. Barrel length is 8" and it's a 5.56 chamber???
1/12 twist should stay at 55 or lighter - try the 40 grain to 50 grain range
as pru stated, the velocity may not be what you think it is. get a chrono reading.
is there even the slightest possibility this is impacting a muzzle device or a suppressor?