Honestly, the problem I have with most current attempts at SMGs is that they are dubiously useful. Automatic is a pretty niche function, since its almost always better to do concentrated damage that might take an enemy out. Doing a weak attack against a large number of enemies ( and then having to spend actions reloading ) is going to generally hurt you versus other ways of fighting. Doubly so if you have any melee party members who don't want to be in the middle of your AoE.
My own design preference would be to give SMGs relatively high damage ( midway between 'normal' pistols and Longarms ), but also a high Usage Rate to go with it. The idea being that an SMG with a clip size of 20 and a usage rate of 4 isn't firing single shots at single targets, its firing bursts of bullets. Then, give them all both the Automatic and the Unwieldy ( 2 Hands ) properties. So, you can use them to spray and pray, but if you use two hands you can also use them to make multiple aimed attacks per round.
