Nomad: To answer your question about whether commercial shotshells would be effective against Mr. Slither, the answer is a qualified "yes". As Nighthiker has already pointed out, you tend to get a donut pattern shooting shotshells out of rifled barrels, even 2" snubbies (I've tested this on paper targets).

While I have never killed a snake with shotshells, not even Mr. Slither whom I met personally, my actual source for the qualified "yes" is my brother in Iowa who regularly mows about 3-4 acres of farmyard infested with all sorts of snakes. He hates snakes and, regardless of all their potential benefits, he doesn't want them in his yard, or around his barns and other outbuildings. When he mows, he always has his .38 Spl Smith and Wesson M&P with the 4 inch barrel on him, and he uses it several times every time he mows to kill snakes up to several feet long (mostly those are bull snakes in Iowa).

Perhaps you've already done this, but if you decide you want to use a shot cartridge in your revolver, I'd suggest that you get a few largish pieces of butcher paper, draw a rough sketch of the size of snake you expect to encounter on it with a magic marker, and then (aiming at the snake head) try shooting a shotshell at a couple of those targets so you can see the shot distribution pattern. That way, you will have some idea how to alter your aim (if necessary) to provide the best concentration of shot where you want it. Even if you don't intend to carry a shotshell in your gun, this is an interesting exercise. My .38 snubby stainless Rossi (which I like very much BTW), throws about a 16" pattern at about 8-10 feet with a slight donut hole distribution of shot. So, if I were wanting to hit a snake head-sized target with the greatest concentration of shot, I would not aim directly at the target but about 4 inches one way or the other.