Hell when i started out in 2.86, i used the MP540/a3 (yes, that old thing) then once i had learned to aim, i cwitched to the M16a2 M203, where a 3 round burst can take out any guy, n once u learn to aim the nades, the M203 i devastating, n this still applies in 2.9
Now that i've been using 2.9 since it was released, and have been experimenting with other guns, i still go back to the M16a2 M203, however, the SG551 and the M4a1 are also good weapons, especially if u use the M4a1 with the Reflex sight, it makes it really easy to aim, n easy to learn how to aim for beginners.
The AKMSU, despite the decent damage n ability to have hi-cap mags, the high recoil n relatively slow rate of fire (compared to other SMG's n 5.56 assault rifles) makes it pretty useless unless in CQB, where the hi-cap drum mags, really shine (for example if u just entered a building through a window, n ur weapon is hipped cus of jumping, then a bad guy pops out u can just point the gun in his general direction n spray, sweeping from side to side, leaving u with a much better chance of survival than a 5.56 pee shooter with a relatively small magazine)
But I advise that u use all guns and get good with all of them so, god forbid, if u didn't take enough ammo with u (u will kno this when u run out) u will b able to take a weapon from a dead guy n use it to a standard where u would stand a good chance of surviving for a while longer, just remember, if ur not being shot at when u get the weapon, get some ammo too!