Get a 7.62. Nothing wrong with the 7.62x39 good for up to 400 yards.
Longer yet get a .308, M1A is what I have built up around, sure its heavier, holds less ammo but its better suited against trained heavily armed insurgents.
My understanding was that the 7.62 is a bit unweildy for a carbine or light machine-gun frame. Kicks too hard. Difficult to quickly re-sight for a double-tap.
SnakeDoc