I much prefer no-legged snakes to the two-legged snakes far more than common in my current AO......... and I'd gladly trade them for rattlers any day.
However, to the original post, you can use a combination load (think buck-and-ball, but smaller). Use 1/3 of the normal powder charge in your cylinders. Use an equivalent volume of #8 shot, seated on top of a very light card wad over the powder, and then load the ball on top, using the ball to seat against the shot.
The idea (for cartridge revolvers) can be found here:
http://www.gunblast.com/Snake_Stopper.htm , and it doesn't take much derivation off that idea to convert it over to the C&B revolver set-ups.
And, yes, they work.
I try to leave rattlers alone (I like them, and consider them to be the true symbol of America), but copperheads and water moccasins get shot on sight.