A modular gun concept with a "naked" gun-base that needs different outer shells to become whole. This allows different aftermarket manufacturers to make different cosmetic shells, while keeping the functional part of the gun reliable. The default shell, seen first below, leaves such options desirable.
On a technical note, I find this 2D approach crucial because of it's speed and flexibility. Would do this to find the feel/concept before I'd consider working out the final design in 3D.