If you want to be able to carry self-defense ammo and rat/snake shot, I'd get a small revolver and load it with the shot, while still toting your primary carry. I have a NAA Mini-Revolver that, while not the most potent, should serve quite well as a snake gun. I've not tried it on a snake yet, though, so I don't know how well the shotshell would perform on a snake out of that little 1 7/8" barrel.
Other options would be a Taurus lightweight .38 with shotshells, or a similar S&W.