My grandfather used to keep an old H&R top-break revolver hanging on a nail in the feed locker of his barn. It was normally loaded with crimped "rat shot" cartridges and worked pretty well inside of 15 ft. or so. Other than the increased hit potential, his main reason for using them was that misses wouldn't damage the woodwork or feed containers as much and the chance of injuring any livestock on the other side of a wall was essentially nil. He also kept a box of SV .22 Shorts there for pigeons and starlings in the calf feeders.
Don't know how the snakes are where you are, but back in SW MO where I grew up it usually took more than one good hit from the old crimped .22 shot cartridges to dispatch your average cottonmouth, or even a much-smaller copperhead. I never (thank goodness) had to try them on a diamondback or timber rattler, but I'd reckon it'd take at least a couple of solid headshots to stop the common run of these sizable serpents. These days I'd much rather rely on something like the .38 Spl/.357 shot loads for any of 'em, as I'm not as fast on my feet as I used to be. When I'm visiting relatives back home I usually have some sort of 1911 .45 along, and pack a couple of boxes of the CCI shot loads in that caliber if we're liable to be fishing, camping or traipsing about in the woods. They'll cycle normally, and pack a lot more wallop at somewhat longer ranges than any .22 RF shotshell I've seen.
