You Need the faster twist to shoot the heavier bullets
Mike,
The original AR15 had a 1/14 twist barrel. This twist proved not accurate enough for the Army during the Arctic Test phase, so they specified the 1/12 twist. This stabilized the 55 gr M193 bullet in the dense arctic air so that the rifle was acceptable for use in all climates.
This was fine until the adoption of the Belgian SS109 bullet as the Nato standard ammunition. This bullet is heavier (62 gr) and longer then the M193. The US Army adopted this round as standard for the M16A2 and the M249 Squad Automatic Weapon in 1985 and designated it the M855. The M855 was most accurate out of a twist of 1/8. Contrary to popular belief, it's not the weight of the bullet that causes it to need a faster twist to shoot accurately, but the length. Even 1/8 twist was not fast enough to stabilize the Tracer round that the Army adopted (M856) which is much longer then the M855 round. So the new family of weapons received barrels with 1/7 twist barrels.
There are a lot of old wives tales floating around that say you can't shoot the lighter weight bullets from 1/8 or 1/7 twist barrels. This is untrue. Some people have experienced problems with very light weight and thin jacketed varmint bullets out of 1/7 twist barrels, but the construction of the bullet has as much to do with that as the twist rate of the barrel.
The lighter bullets will be just as accurate out of the faster barrels but the zero will change. I/9 is a good compromise, fast enough for the longer, heavier bullets but not so fast that fragile varmint bullets will break apart in flight.
HTH
Jeff
