Snakes are pretty hard to kill with a single projectile weapon. Even copperheads basking in the sun are going to be hard to kill because their brains are so small. A shot most anywhere else will produce a wounded snake, which may make it more agressive toward people than an unwounded snake. If you've got a full blown snake problem, consider an air shotgun or snake shot in a pistol, or a .410 single shot with small shot.
I lived among a lot of venomous snakes in the south for many years and they were never a problem. The most agressive ones were the cottonmouths, and even they avoided people. They were just less shy, particularly at night, than the copperheads and rattlers.
