I had to dispatch a terminally ill critter at lunch
Not something I like to do at all. I even swung by the feed store for medications in case it was not as dire. But it was heading into shock, in distress.
I have used several arms for this purpose but I find the old .22 LR HP to be as effective as anything and less of a mess. There is the old tried and true M63 S&W I have used, even on a couple of deer that had been hit by automobiles. This time I used my Ruger SP101 .22 8-shot. It's been my go-to for a while. It was handy, while the S&W rests in the safe. The .22 WMR is so loud from a handgun and at 5X the cost, it's simply not needed for this work. On really big animals though I'd certainly find it superior. Last horse I had to put down I used a .45 ACP in an IWI Jericho, of all things.