Once upon a time...
The original AR-15 (Back when AR meant Armalite Rifle and not ARmalite™) had a 1:14 twist to the rifling.
The R601 went with Project Agile to southeast Asia and was well regarded.
However, it was quickly found that in the denser air that comes with air that's not tropically hot and humid 1:14 was insufficient to stabilize the bullet that would become 5.56mm M193.
Thus the rifling was changed to 1:12 and the model changed to R602.
It's not the only time 5.56x45mm has had this sort of issue.
Later when Fabrique National was developing what would become the SS109 round and establish the NATO standard for 5.56; 1:9 rifling was specified.
Temperature and humidity came into play again when it was discovered that the L110 tracer version of the round was unstable when you put it in arctic weather, so 1:7 was specified for tracers.
The US portion of this caused a bit of debate. 1:9 was sufficient for ball, but not tracer, so it was conceivable that the new M16A2 could have the slower rifling than the M249 because rifles hardly ever fire tracers. Ultimately they decided that any 5.56 NATO weapon should be able to fire any 5.56 NATO round and thus the rifles and carbines got 1:7 twists.
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