I have annihilated scores of chipmunks with a .22 mag. using shot. And, yes, I have killed a rattler and many copperheads with the same medicine. But, that is .22 mag. I don't know how regular .22lr shot performs. You do have to be very close since there isn't much shot in that capsule.
On my farm I carried my .44 mag. with shot capsules I loaded myself and used that for mostly copperheads. I used #7 1/2 shot in it and that carried much further with killing effect.