Shell by shell reloading. Also, I think Wolf's kickback idea is very good... but I do think that when shooting from a point blank range it should still kill with one shot. I like the current spread and number of pellets, just bring the damage down a bit.
EDIT: Come to think of it, actually... how much damage does the shotty do in one hit? If you can't bring down the damage enough to make it balanced while keeping its ability to one-shot kill from up close, I guess you'd have to adjust the spread at least a bit.
EDIT: Come to think of it, actually... how much damage does the shotty do in one hit? If you can't bring down the damage enough to make it balanced while keeping its ability to one-shot kill from up close, I guess you'd have to adjust the spread at least a bit.