I'm looking for far more specific subsystems than just the generic under-barrel. How about:

Reciever Slot: can modify caliber/rate of fire (no restrictions on matching caliber to weapons though you might not be able to have a magazine for a pistol shooting .50 cal sniper rounds, it would be limited to a hillariously high powered breach loading weapon with no mag)
Mag slot: can modify mag size and certain sized mags will only work with certain sized recievers
Stock slot: this would modify accuracy, stability and movement speed while zoomed but might also raise ammo capacity (options to include but not be limited to full, cut down, none, folding, etc...)
Barrel slot: would modify range and accuracy at the expense/benefit of stability. Longer barrels would increase range and accuracy but decrease stability, or barrels with no rifling (ie smooth boar) would decrease both range and accuracy making a scrambler pistol into a type of shot gun)
Under-Barrel Slot: All sorts of nifty add-ons to your weapon could be installed here from a light or a laser sight to a forearm grip and a grenade launcher. These options could add secondary weapon functionality but might modify weapon accuracy/stability (in a positive way in the case of forearm grips and tri-pods or a negative way in the case of grenade launchers or a under-barrel shot gun <-- because of the added weight of course).
Equipment Rails: These are on the top of the gun and would be used to modify the base iron sights with scopes/red dots but this would be done at the cost of stability in the case of large attachments.

All of these slot options may or may not modify pg/cpu depending on the options selected but all of them will add to the price of the weapon. Remember that weapons of this nature would be inherently high level (tech 2 or 3?) since even pretty experienced firearms users don't typically modify their weapons' recievers. By setting large skill and possibly ISK barriers to weapon modification it would become a motivating goal for many Dusters but would not become a super dominant item on the field. How long will it take people to skill up to the point to meet the type 2/3 requirements after all; a while I'd hope. Also because of the sheer number of combinations people would probably use these mods to make pretty specialized weapons with reduced general utility.

Sorry if I'm a bit rambly here. I'm tired. I'll take the time to clean this up and make it more sensible (especially that last paragraph) later tonight when I am more functioning. This is the problem with posting during breakfast.