Bullet tumble/keyhole

Bad ammo is certainly a possibility. The fact that it was a light bullet is not an issue though. A good 55gr bullet will shoot just fine in a 1:7 barrel, just like it will in a 1:12 barrel. Back in the old days, when bullets were not made to the tolerances they are today, the theory was not to "over twist" them. If the bullet had a flaw, supposedly spinning it faster would increase the flaw. That's why many benchrest guns barely have enough twist to shoot their intended bullet. It's been proven though that modern bullets of good manufacture almost can't be over twisted, unless it is a super lightweight bullet at high speed that comes apart midair. In short, 55s in a 1:8 is not the issue at all.

Cheap bullets of questionable make don't shoot well in most barrels, regardless of the twist.

 

Alittle late to add but throat of chamber is tad longer on 556 chamber, so will shoot 223 also!! but no 556 in 223 chamber!!.sounds like ammo. try heavier bullet and definitly better quality ammo

 

Just to chime in, DO NOT shoot 556 out of your barrel if it’s a 223. Also, I’ve shot about 2500 rounds of tulammo steel case out of my F1 ar-15 (bought in bulk about 26 cents a round) and have had no issues at all, YMMV.