it will be 10 times less hassle to breed them -
freezing does not necessarily kill all parasites - it can put some parasite eggs into stasis which will hatch when re-heated, similarly, you have no idea of previous infectious diseases that wild rabbits have - there's been some pretty weird biological warfare against rabbits over the last decade - just because the rabbits have managed a partial immunity to most of these diseases doesn't mean they wont adversely affect your snake (short or long term)...and as chantelle said, people regularly use poison bates that can take up to 3 days to have a fatal effect on the rabbit - i guess i'm saying that it's not worth the risk - especially if your snakes are highly bred or bred over a long line and don't have much of a natural immunity...
and you absolutely CANNOT kill rabbits humanely with a slug gun - you'll need a .22 or a .223 - you'd have a more humane attempt with your car keys than a slug gun - i got shot by a slug in the shoulder and it barely went 5mm deep.
A decent sized elliot trap would be good but the whole process is really much more risk than it's worth - you're better to breed the rabbits, just make sure you have something decent to kill them with and remove the bullet immediately to stop the lead from spreading through the blood