I had a 7.62x39 in a Ruger mini 30, but seriously, with that pencil thin barrel, after the first two shots the groups opened up enormously. I sold it.
I've an 870, M1A, and an AP4 .308 panther. : )
The rifles now referred to as “assault rifles” came to be in the latter years of WWll, when the Germans found that the full-house infantry round, the 8x57mm Mauser, was uncontrollable in a rifle-weight shoulder-fired weapon. The devised the MP-44 “Sturmgewhre”, a rifle featuring select fire and firing a less-powerful cartrige, the 8x33mm. Mikhail Kalasnikov applied the concept to his AK-47. The definition of “assault rifle by the military is a select-fire rifle of INTERMEDIATE POWER.
The M-14 was unmanagable in full-auto fire. The old BAR was capable of select fire but tipped the scales at 20 lbs.
I have seen AR-15s used in high power rifle competitions at 600 yards but it requires an 80gr. bullet with a different rate of twist in the barrel rifling. There is a difference between punching a hole in a piece of paper and making certain that SOB 600 yards out is down for the count. The M-4 is a cut-down M-16. In making it more compact for urban warfare, a good deal of velocity was sacrifices, further reducing the effectiveness of a marginally effective round.
It is difficult to build a rifle that will do all things for all people but in the end, ya gotta have enough gun