I know this is an old thread, but I'd like to address the "paratrooper" aspect of rifles.
I was on jump status for most (all but two years) of my 24 years in the military between the Reg. Army and National Guard. I jumped with a lot of rifles including the Garand, M-14, M16 and even a M700 Rem sniper rifle.
If you cover (tap over) the muzzle of these guns, they are all "ready" when you hit the ground. I never used a weapons carrier with a rifle (have with a M-60), they are not needed. If you jump with snow shoes, or skis you'll discover the rifle isnt in the way at all.
I'm infantry, not armor, but our mech units were carrying Grease guns until we went to the M16s so I never really saw the need for the "tanker" model, I don't know of any USGI models.
I played with one and wasn't impress, nothing it would do that a standard M1 wouldnt, but the standard was more reliable, and more accurate. (Longer sight radius has a lot to do with that).
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071