And remember, the original rifling was 1/12. The army was using 55 gr bullets that were much more rounded, more like a football. They were intentionally making them yaw, it was very common to get sideways imprints of the bullets going through the paper at longer ranges. When they switched to the A2, they also switched to a 62 gr bullet, which is longer and the edge that touches the rifling (the ogive) is flat. The reason for 1/7 rifling is that tracers need a bit more stabilization than regular rounds. (I have fired tracers in machine guns, but never in an M-16.)
In my experience, 1/9 handles everything very well, including 55 gr bullets. Particularly Barnes all-copper bullets, because they are a bit longer and flatter than lead bullets. The twist rate has just as much to do with the SHAPE of the bullet as it does the WEIGHT.