Hmm...wait here. I'm going to try something, and will edit this post when I finish it/fail it.
EDIT: I didnt get past "how do i edit this". There is this weapons mod called WWHC_Diaz. And it is a great one. it has about 2 weapons for each slot, each something different. It also has random replacements for actors, so instead of 20 imps, you might get some crazy scientists with genetic enhancers, some heretics, and a few imps.
Well, the one I was looking for was the Mauser Rifle. It replaces the SSG, and does a mean bit of damage. it has an ALTERNATE fire mode, in which the sprite set changes to show the difference between firing from the hip and firing from the shoulder. I think, as far as i can tell, it uses "sister weapons" to accomplish this. This gun, however, has major kick along the mouselook axis, and also major kick backwards. Along with the fact that you run move slower with the gun in your hands, even SLOWER with this gun on your shoulder. The hip shot is more powerful, but is random as you quickly raise it up. The shoulder is less powerful, but has a better chance of going where you want it. Shall I post a DL link or a section of code for you?