I have once successfully used .22 snake shot on a 4' Copperhead. Fired from a 5.5" Heritage single action. I was about 6' to 8' away, approached from behind, and picked an aim point immediately behind his head. There was some muscle twitching, but he was dead. To be effective with .22 snake shot, you need to be as close as you dare.
OTH, if you live on a ranch, do you have neighbors close by? You could always just use standard .22 ammunition.