What caliber for cats?
Not sure on the caliber, but the rifle would have to be a Mouser!
Actually probably what my mother always used to use, a .22LR. Single shot JC Higgins bolt gun, and she was deadly with it. We always had lots of cats, normally a half dozen or so, on one occasion when three queens littered at the same time we had nearly 30 cats and kittens. They were "outdoor cats mostly, with a few allowed in as pets. When it became necessary to destroy one, nobody loat any sleep over it. Sometimes they would get sick (mange) and had to be put down. Distemper took its share as well. So did the highway. Vets were for cows and horses (things worth the expense-and then rarely, as we didn't have much money)
Mom also had a firm rule. The third time a cat crapped under her bed, it was obviously too stupid to live, and it didn't for very long after that. Many times I would come home from school and find Mom leaning out the upstairs window, waiting for a particular cat to come into range. Only thing I didn't much care for was when I got big enough, I got the chore of burying them.
Don't take it wrong, Mom loved cats. I spent one whole day tagging along as she searched every store in the city searching for a doll's baby bottle, so she could feed some week old kittens who's mom got killed by a car. A year later she had no trouble shooting one of those same kittens, who grew up "defective". Mom was fair, but she had her standards!