As I've said before, I've dicked with the recoil on the SMG and if you turn it up too high it just becomes unusable shit. (EDIT: I've said the first half before, although not really the second half. My bad.)
Anyway, here's an idea for the shotgun:
Basically, cone-sphere intersection, then finding the area between two circles, and dividing that by the area of the cone. This will get you your damage falloff in the range [0,1]. Multiply that by your intended damage, and there you go.
In other words, cone = shotgun spread, and sphere = your head.
To make stuff interesting I think I might just apply it to everyone instead of having that bug where you shoot player #11 and kill player #8 who happens to be standing right behind him. (now WHY have I not heard of this bug until now‽‽‽)