I have the same experience as several of the others;
.38 and .45ACP shotshells are quite effective on snakes, I have 3 in the freezer that fell victim to a 45 from 20 - 25 feet.
On the other hand, I once emptied a 9 shot revolver into (at?) a rattler from less than 10 feet with no effect, and I don't want to be any closer than that.