Driftrider,

When we transitioned from the M16A1 to the M16A2 the Army still had a gazillion M193 (55 gr FMJ) rounds for the M16A1, which had a 1-12" twist. The M16A2 had a 1-7" twist for the heavier M855 round. The Ordanance guys told us that there would be some degradation in accuracy at targets beyond 300m (330 yds). If you're target shooting at 100 yds, then surplus Mil-spec ammo should work fine.