I have one of these in .22, I bought it as a backyard plinker and a bit of a challenge, took it apart deburred and relubed, very easy to do and the result a nice consistent 4.5ft/lb plinker. Personally I'd never need to use it for humane dispatch but would think it'd be up to the job at point blank or nearest offer.
At the risk of stirring up a shit storm, drowning? Not necessarily my point of view just thought I'd put it forward for discussion. I remember reading a good while ago the RSPCA tried to have a guy charged with cruelty for drowning a grey squirrel, not sure how that ended up but from a legal point of view this would probably set the precedence. From an ethical stance there are arguments both for and against, there are those who believe drowning is a very peaceful way to go.