I've killed rattlers in WA out my inlaws place with my Blackhawk in 45 Colt.
I load the Speer shot capsules with #8 or #9 shot.

I always carry the snake loads when at the range on their place, especially when the grass gets tall in mid-late summer. The snakes love to lay in the shade not too far from the creek when the temps go up. A couple loads of shot always puts them in their place:)