Mike, if you are limited to using 12 ga. (way more than needed, really) why not just use 8 1/2 birdshot or target loads.... cheap, available, and will certainly get the job done.
Or .38/.357 snakeshot in a revolver of some sort?
I had a western diamond back rattler on my back patio at my desert home in Scottsdale a few weeks ago. Hated to kill him, but didn't want my wife or dog to ever encounter him again. 00 Buck already loaded in the little coach gun that I keep close by did the job swiftly and surely. Overkill, of course, but you use what yas got at hand at the time!
If I had regular use for it for snakes and vermin, a S&W Governor with .410 birdshot might be a good resource.