I bought .38 shot shells to use for snakes in my 6" bbl. GP-100. They gave me a false sense of security for a while.
I finally got curious enough to try them out on some milk jugs. I was amazed at how few pellets (like 2-3) placed on a gallon jug from 6' away. Frankly, I lost interest in them after that. I do not plan to be shooting at rattlers from closer than 6' and my impression was the shells were useless at any greater range. Maybe it was just the long barrel, but they were not a reliable snake-stopper in that rig, which is still my favorite.
The only snake I ever actually shot was with the dreaded Black Talon .357 slugs, which were in there for totally different reasons. Took some aiming at 20' or so, and it was, of course, ridiculously massive overkill on a fairly modest little rattler. It was while prospecting in back country and the snake had all my stuff (which I dropped when I almost stepped on it), or I would've let it go.
Now I try to use air guns for everything and I would be good at 15-20 yards on a rattler head. If he is closer than 6' I am moving, not shooting. On my property, I'd just mark the spot, fetch the Marauder, and put a nearly silent .22 through the brain.