I've killed dozens of snakes with .22 shotshells. Shots taken at four feet or less kills. At over four feet I have had to shoot them four or five times. I've also used .22 magnums, .38's, and .44's. The .22 magnums work a little better but not much. The .38 and .44 will turn their lights out. (all shots being head shots) I am now using a .45 acp with shotshells, works good but does not cycle my auto.
I agree with others about leaving them alone when possible. But if they are found in the yard they are killed and any kind of water snake is also killed.
Dallas Jack