There is some misunderstanding of the rifling situation evident. The original M16 rifling twist was 1:14, which was OK when using the M193 round with the 55 grain bullet. But it was soon discovered that bullet stabilization was poor under very low temperature conditions such as found in the Arctic, so the twist was changed to 1:12 which solved the low temperature problem. However, in the meanwhile the M249 SAW appeared, and for it to reach full effectiveness at longer ranges, a heavier 62 grain bullet with a steel penetrator was developed (M855). But that created another logistical problem having two rounds (M193 and M855) in the supply system which were not fully interchangeable, as the 1:12 twist of the M16 barrel was inadequate to stabilize the heavier M855 bullet. So around 1982, the decision was made to go with the M855 round as standard for both the M16 and the M249. But of course that required that the M16's barrel be changed to a faster twist, 1:7, to achieve bullet stability when firing the M855 round. Actually, a 1:9 twist is adequate to stabilize the M855 (62 grain bullet) round (and even heavier bullets). However, a 1:9 twist is inadequate to stabilize the M856 tracer round, so the choice was made to use the 1:7 twist for the M16 and M4. And so it remains. No problem in using the early M193 round in a 1:7 twist barrel. I'm not sure the old M193 round is still in production for US military use, but I doubt it. Up until around 2006, the USAF used M193 ammunition for training, but then adopted a lead-free frangible bullet training round for that purpose. The current round is the improved M855A1 round which reportedly produces somewhat better lethality performance against both soft and hard targets than the original M855 round. But that is another story.