For cotton tails a rifle chambered in 22 LR would be practical and obvious. Won't wash.

BTW: While a 22 LR will knock down a cotton tail, jack rabbits are another story. I've seen jacks hit fair and square with a 30-06, 455 Webley, etc., and keep going. I've also decided a 22 LR is under powered for wood chucks after shooting one through the boiler room and not getting a quick clean kill.