I've only shot one rattlesnake with snakeshot and it was at about 1 yard. Ended up holding the snake's head down with a stick and cutting it off with a knife to kill it, The shot had little to no effect on the snake so I quit using it and went back to real ammo.

Oh, and this was with a .38 snub nose. Maybe if I had a 20 inch barrel it would have worked better, in other words a rifle.