Since none of these weapons are "real" but just archetypes... drop the ammo on shotgun to 8 and the ammo on the sniper rifle to 12 (or 9 if it's heavy). Make ammo a little more precious and give the pitiful AR an advantage with it's clip size... I haven't had to reload yet in game (aside from the rocket launcher).
I still think burst on AR sucks.
It never/rarely hits more than one shot at range (realistically) so I'm better off using a sniper aimed shot for more damage.
It never/rarely hits more than twice at close range, so I'm better off using a shotgun and firing two shots for more damage. Even if it hits 3 times the only real bonus is there is no chance of reaction fire for 75 damage vs 35x2 on a shottie.
I suppose it still can do both roles, yet not as well, so it has some value, but it's really not growing on me aside from doing a lot of aim level 2 potshots.
If you are going to make it a heavy weapon, it should do more damage (50 cal?) and probably take a tiny bit more TUs. It might be worthwhile to drag one of the previous sniper soviet models out, because I can't see it being a heavy weapon if it's just an AR with a scope and longer barrel.