A very quick guide in how to draw a typical “body” for a weapon. This is a very classic design that you later can add different kinds of barrels and stocks to make it either a big gun or a shotgun etc etc.

Starting with the handle on a gun is just as recommended as starting with the spine on a human body. As long as you have the handle and main body, you can add other stuff with your own imagination - or mix it with other references.