I had this idea when playing star wars republic commando. now if anyone has played the game you'd know the coolest gun in the game is the default gun. it's a laser rifle looking thing with two attachment slots and three sets of attachments.
so without any of the attachments it's just a hunk of metal but put on the sniper barrel and sniper magazine and you got yourself a sniper rifle, put on the assault barrel and assault magazine and you got yourself an SMG/assault rifle, finally you put on the explosive/anti armor barrel (there's no magazine you load it from the back, it's a single shot sort of deal) you have a grenade launcher.
i do know deep down in my heart this is awesome, i love utilitarian modular weapon systems that can be primed for any situation in a matter of seconds.
you could add a function to the pie menu to change attachments and the character using the weapon would go into its reload animation (last i knew when you reload your character's off hand spins around) while it swaps out parts.
what would be insanely awesome is if we could get the AI to use the different attachments as well. i'm guessing you could set up parameters so like when engaging an enemy at normal range it uses the assault attachment, at long range the sniper attachment and against groups or heavy troops/armor the explosive attachment.
just an idea
oh, and ps, i like bullpups, so if i just gave you this idea and make it a bullpup if you wish plox
