For fully automatic weapons like the SMG (which i think is 9mm), Ak-47 and Assault rifle i love to put at least 2-4xscope and Semi-trigger thingie. And a mag extender/drum magazine if available. Saves me a lot of ammo. So much, i didn't have to craft a single round before day 50. For pistols i prefer the 2xscope and a mag-extender.
I didn't find the SMG early enough in my last play, and in the end i just had an assault rifle and 2 stacks of 7.62 ammo with me when out looting. And a turret in case i expected tougher resistance.
edit: One thing i would really like them doing, is one way or another list , in game, which mod fits which weapon. Either on the weapon, or on the mod.