Originally Posted By pavlovwolf:
It's either the flash hider, or there was a problem with the barrel, or, one of the ammo types in question is undersized. Remove the flash hider, then, one by one, fire a box of each ammo at a separate target. If only one ammo brand shows the problem, bingo. If they all do, it's the barrel.
If none do. Reinstall the F/H, and repeat the test. If they all do it. It's the flash hider. If one does it, it's that ammo.
55gr will shoot fine in any 1/7 twist. It is overly long bullets that have stabilization problems with 1/9-1/12 twist barrels. It's the length and not the weight, but since the heavier bullets are normally longer, weight is associated with the symptom. There are heavier, shorter bullets that stabilize in the slower twist barrels just fine. What you have with a fast twist like 1/7 is over stabilization. Normally this tends to cause a round to self destruct if it's construction is like that of a varmint round.
I've never encountered a 1-7 that would not shoot 55gr OK. I do get key holing from longer (heavier) bullets in my 1-9 barrels.