Once you acquire those shot shells, it's worthwhile doing a pattern test. Get a large piece of cardboard, or maybe some butcher paper, and try them out at different distances. I have some in .44 mag (4" Anaconda) and .45 Colt (4" S&W Mountain gun). They are both OK out to about 8', and no more. I prefer the Mountain gun due to the much lighter carry weight.
At any distance over about 8', the snake is relatively safe.
When I was a teenager, my two younger sisters were in the county 4-H beef club, and I was their designated mice-in-the-corncrib shooter*. I used my .22 rifle, and was relatively inefficient at it. I patterned my loads, and found just what Igor did. Apparently, the shot load travels up the bore as a clump, engages the rifling and is spun by it, just like a bullet, aquiring cetrifugal force. Once out of the confinement of the barrel, the centrifugal force acts to throw the shot outward. This leaves a hole in the middle of the pattern, and the paradox of the better your aim, the more likely you will miss.
* Also hay bale pitcher in the haylofts, feedbag humper, truck and trailer driver, water hauler, et c.
Many years later, while perusing the Numrich (as they were then, pre-"Gun Parts Corp") catalog, I saw a listing for a smoothbore barrel for another .22 rifle which I had since aquired. As I live about 20 miles from there (I practically used to live in the old retail store), I went over and picked it up in person, as well as another one of that model (Stevens 87) .22, so I wouldn't lose a good .22 rifle by the deal. I installed the barrel, and I finally had the .22 shotgun I had wanted all those years ago. Have also wound up buying the odd .22 shoothbore at gun shows- seems they were somewhat popular in the '30s and '40s due to the cost/lack of availability of regular shotgun ammo for sport shooting during the Depression and WWII.