As far as ammo for snakes, the best idea is snake or rat shot in pistol loads. I've seen them in .22, .38, and .45, and I'm sure they come in others. On rocks, anything will (not might) riochett and may put a hurtin' on YOU or someone around you. I have also seen .410 Derringers (really small pistols) at gun shows that would be handy for snakes. At a minimum, wear some sort of safety glasses (many quality sunglasses are polycarbonate.) Solid shot (.22, .38, or .30-30) should not be fired around rocks.
Interesting side note, I saw a 4' snake (king, I think) climb a tree practically straight up, with one small kink in bark, to a limb 4.5' off the ground. I was impressed.
