I have used them in a 44 mag with #9 shot. I do not recall having any problems with them backing out. I just gave them a good crimp over a light charge of Unique. I do not care which variety you use (357, 44, 45) they will develop a hole in the middle of the pattern fairly quickly out the end of the barrel. the faster the twist of the rifling the quicker it puts a hole in. They should work just fine for a few feet on snakes.
A few years back I was having trouble with some thing getting into my garbage. They (it) would tear open the bags and scatter it all over the place. Pick up was on Wed so I would put it out before I went to work and when I returned I had a mess. Believe it or not the pick up guys would not go through the yard and pick it up for me.
This went on for about 5 weeks. I am getting a little trimmed by now about it. The following Saturday, I loaded up 5 shot capsules in my old Virgina Dragoon, put the trash out, and sat down to have a cup o' Joe. It wasn't too long and three dogs started to sniff around the trash bags. I went to the front door and stepped through and I shot three times in their direction from about 30 yards. There was a whole lot of yipping and dogs going every where. I brought the sacks back in the garage for the next Wed pick up. Next Wed set out the trash as always and when I came home, no mess or fuss. The dogs did not seem any worse off then they were before and they never bothered the trash again.
Good Luck and Good Shooting