I really like these sprites
But I'm going need to mod this mod before I can use it.
Have you considered lending your work to complement a major mod? I'm seriously going to jack this into Area51...
But as for my opinions!
The shotguns are hilariously OP because the mod isn't designed to work with OXCE+, so they don't have access to the accuracy bonuses included in that. I admit they are a bit metal on the alien side of things (have fun facing the plasma shotgun) but I feel like that helps to balance the game some at the same time.
Yeah, I will have to adjust the shotguns to work with UFOextender accuracy mod. Right now your shotguns wipe out enemies even at mid range. Which eliminates the need for rifles and such.
The heavy machine gun is meant to be a middle ground between the rifle and the auto cannon. I will freely admit that it fills its role too well, but the damage drops off pretty heavily in the mid-game when the Mutons start showing up and, like most weapons, it's more or less rendered useless against Sectopods. (HMG-IN?)
The way Area51 balances out the machine gun vs the AC, is that the AC keeps it's unique ammo, and has better accuracy in all modes. In return, the MG does less damage, and has no aimed shot-but has a very cheap TU cost (16%) for firing extended bullet auto-shots. Essentially the only way to make them really unique from each other is to make the MG a primary auto-attack weapon. Of course this requires UFOEx accuracy compatibility, which might be beyond the scope of this mod.
The stun rounds were meant to take the edge off capturing aliens. I admit they do allow for a lot of other utility uses as well, but given the variety of alien weapons you'll be up against when playing, you might be better off trying to stun them from afar than you will from exposing yourself to get clear shots.
I think the stun damage is fine. I actually used a variant of this weapon in a TFTD Mod, it was useful for clearing out the field, but not so much in close quarters. It was also much less effective against any aliens higher than Aquatoids, due to the fact that most aliens in TFTD seem to have better armor and HP than their XCOM UFO Defense kin, and the base damage was slightly lower.
On the other hand, money is so tight in that game I needed to capture aliens alive in vast numbers just to pay the bills...
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Also, have you considered adding SolariusScorches Plasma Blades to your mod? That seems to be the only thing your aliens lack, melee weapons with which to kill XCOM even more efficiently.