A number of years ago, I experimented with a variety of ways to make shotshells for my revolvers. I ended up using
Speer shot capsules in .38 Special, .44 Special, and .45 Colt. I found that Speer's suggestion to NOT load at max gave better patterns to be correct (well, duh-h-h).

The real break through for me came when I was able to buy a small supply of #12 shot from Ballistic Products. The patterns were MUCH better and at the ranges needed for poisonous snakes would absolutely shred a snakes head. Since the ranges are so close, there was NO problem with penetration of the small shot. They were EXTREMELY effective. They also worked very well on rats in a barn. Further, the small shot were not apt to damage anything in the barn, so another advantage.

#12 shot is expensive but you can buddy up with three or four guys, split the cost, and have a lifetime supply if you are just going to use them for snake and rat control. I used to do a lot of hiking in the mountains and the small #12 shot was also safer to use on rocky ground. It was less apt to bounce back and do serious damage to the user.

FWIW
Dale53