I have been in the woods most of my life and in all that time have only dispatched a few snakes. For the most part I give them the same chance they give me, I leave them alone if they leave me alone.
That said fishing was a major part of my up bringing, and as such there were the occasional surprises like pulling up the fish basket to find a cotton mouth hanging half out of it with a fish stuck halfway down to it's belly, or raising up a trot line to have one attached firmly to a wiggling catfish. Neither of these in the pitch dark are much fun to deal with.
For the others like this huge copper head I caught behind one of the sheds on our farm, I simply packed them up in a pillow case and relocated them to fend for themselves against the hogs, in the swamp down back.
Seeing as most copperheads I have run across in my close to 50yrs of being here have only been in the 2' or so lenght it was out of respect I let this big ol gal go. She had to be pretty old to have gotten to the almost 4' length, and I figured she had kept plenty of rats in check in this time.
As for the rat shot, I got on a kick quite a few years back and worked up loads for both my .357 and my 44 which worked really well, well enough for rabbits and a couple in air birds. With the 357's I found that using shot in the 4-6 range worked the best from my GP100's and with the 44, well the same applied, but the below ones were my favorite.
Loaded using the data on the box they were great fun and plenty accurate to 25yds for nuisance critters like raccoons and such, and if needed would easily take out a snake at close range. They use two f the Speer .350 diameter round balls, and the felt is simply used to keep everything tight in the capsule.
The only time I have felt the REAL need to have dispatched a snake was once while spring turkey hunting up in the hill country. I came down a granite outcropping while trying to get around and cut off a flock which was working down the hill 50 or so yards out, and had stopped to hit the call. After a couple of chirps I heard this buzzing sound I couldn't quite figure out until I started looking around on the granite. There in the 5-10 yards around me were at least a dozen or more rattle snakes in the 1 -2" length range all out sunning on this particular rock. Having my 357 with me, was of no good as it was loaded with 140gr JHP's, and even though I dew it I quickly realized the error had I decided to drop the hammer. I tip toed out of the area and decided the turkeys weren't quite so important that day.