I've only shot one snake with .38 snake shot (the CCI .38/.357 stuff), and I was pretty underwhelmed. Fired two rounds at a distance of 6-7 feet. The snake was right in the center of the spread with both shots. He hightailed it out of there, but I don't know if he died later, or if he was just sore in the morning. I've had much better results with .22 hollowpoints!
If I use snake shot again, it'll be either .44 or .45 Colt.