Multi-caliber weapon system

A multi-caliber / multi-purpose weapon system that would allow the user to assemble the gun in four different configurations: as a smg, a carbine, a rifle or as a marksman rifle. Calibers 9x19, 5.56 and .338 LMagnum.
My original intent was to make the switchable parts separate, so the end user could then build the weapon they want, but because of time limitations and forseeable problems on that route, I decided to model just pre-constucted weapon variants.