.410
Well, yeah, here's your ultimate snake defense gun of course:
http://www.taurususa.com/products/product-details.cfm?model=4410TrackerB&category=Revolver
But until then.... I understand that 99.9% of the time snakes are not a problem - they will go the other way. But cottonmouths are the rare exception that sometimes will be very aggressive/territorial, and when you're up past your knees in water, you're at a great disadvantage in escaping vs. an animal who swims very fast on top of the water. There's quite a few cottonmouths where I have bowfished before. I'm gonna run with the .45 with handload shot until I get the taurus .410/.45 colt.