I think that the .22 CCI Shotshells are fairly effective at no more than about 2yrds & out of a short barrel gun like a 317. I killed a lot of Copperheads with this rig & ammo.
The other Posters are right imo about being careful to aim right at the head as if you are shooting a single projectile.
Also, like others have said, even though the snake is writhing and flipping around, that's just what snakes do when they expire. That's just normal & they think the shotshell didn't work.