While I don't dispute that one can exhibit the skill necessary to kill snakes with a 9mm or .22LR, or even a .177 pellet gun for that matter, I should point out that shooting a non-coiled-up snake and actually hitting it is easier said than done.
You said a mouthful.
The last snake I shot with a .45, I had to shoot at him NINE times before I hit got him. That was the worst I've ever been.
In my opinion, the best firearms for snakes are going to fall into the lines of revolvers using shotshells, a short shotgun, or a .22 rifle.
The ones that I've used have been not ideal, and overkill. But the reason is that I carry a firearm for more general purpose, rather than specifically snakes when I am out on our land. If I see one, I make do with it. If I use a handgun, its most likely because that's all I have in the jeep or 4-wheeler at the time.
I am exploring the idea of a short shotgun in the lines of a coachgun as possibility. As it is, my shotgun is a bit too long to "knock" around with.
If you live in the "woods" and go shooting up all the snakes, next year you'll posting, "What caliber for mice and rats?".
For me, it hasn't happened yet-- and that's after years of killing cotton mouths. I suppose we still have enough predators for rodents around here. I'll take a rat problem
in our woods over poisonous snakes any day.
After all, cotton mouths are aggressive. I've had them "run" at me, and I've seen them strike without being in any obvious danger. They're just a nasty damned snake. I've had one kill an 80 pound Chow from a neck bite. It isn't going to happen to my JRT's if I can help it.
-- John