The only problem I had with shot shells were they didn't kill our big rattlers (from 3 to 7') in the WV mountains) very well until you got to 4-6 feet.
I've treated around 20+ rattler bites. plus a few copperheads, and have seen the damage they do and the misery they cause even with proper treatment.
So, my standard snake load is a HP in .44 or .454 to the head. No head, no bite. I'm not getting that close to these big boys.