Scenarshooter: I have used my S&W Model 34's (2" and 4"), Model 63's (2" and 4") and my Model 651 (4" 22 Magnum) for killing rattlesnakes for a LONG time now.
I prefer the shotshells.
I imagine I have killed 25+ to date with these arms and shotshells.
Again I prefer the shotshells to slugs - as a general rule it takes up to 3 shots from 6' or so with the 22 L.R. shotshells to quickly (humanely) kill said snakes.
Where as I generally just shoot just twice with the 22 Magnum shot shells to render the snakes inert.
Hold into the wind
VarmintGuy