How about the ability to adjust the fire rate and clip size of guns? You could adjust the clip size to a maximum of 30 for SMG, 10 for rifle, and 6 for shotgun. We should also be able to reduce the firing rate of certain weapons (SMG/rifle).
This would allow versatility for weapons. You could reduce the fire rate of the SMG if you want to conserve ammo and regulate recoil.
Modders can also adjust the clip size of a weapon to it's real-life configuration. All modifications, though, only take effect on the person using it.