I trained with the M1, M14 and M16 (ok i'm old) and vastly preferred the M14 to the M16, which I thought was a bit flimsy, and didn't throw enough of a projectile at the bad guys. I grew up with the marksmanship/hit what you aim at mentality, where ammunition was a precious resource. Still think that. Of course I have a black rifle now, in 5.56 but might really prefer it in 308/7.62 (308 so I could use both).
M14 initial selection was complicated, probably dominated by "fight the last war" mentality including "ya gotta have a bayonet", use a 30 cal, but further complicated by the fact that with so many draftees in Vietnam, the ranks did not include many people with marksmanship and woodcraft skills. All this treated the soldiers and their rifles as disposable. And of course, in the jungle, long distance shooting was a rare thing; spray and pray. Although those tiny little 5.56 projectiles were easily deflected by foliage. They are mostly a wounding cartridge with its tumbling, which has an added benefit of weighing down the opposition to care for casualties. It may have been FMJ but it behaves like an expanding bullet in some ways.
The Germans had the STG 44 toward the end of WWII, shooting a 7.92mm Kurz cartridge a vastly superior mid to short range battle rifle to even the M1, given it's lighter ammunition and magazine feed. Kalashnikov stole the design; we should have brought it over along with the Operation Paperclip rocket scientists.