Clarification. First velocity figures are actual 20" MV...

by passing by, Friday, May 24, 2019, 18:30 (1799 days ago) @ passing by

not NATO measured at distance, and not SAAMI 24"bbl. What the Army wanted right off was their 20" to equal a 24". It makes a difference in tumbling/fragmentation, moving limit from circa 100yds to 150yds.

Of course, then they went M855 heavier bullet putting them right back to 3100fps in a 20", then shortened barrel, taking them to under 2900fps, and boosted pressure (and cyclic rates). And wonder why it broke, with bolts trying to unlock while chamber pressure still high in carbines.

It needed improving, but they improved it for wrong reasons, to operate more and more outside design parameters. Shoot a new and improved material processes bolt, extractor, etc, in 20", using .223, and you have one heck of a modern gun.

Expect shorter service life with any other change or permutation.