Caliber .22, whether .22 LR, .223 Rem, or .22-250. I could live with only .22s if I had to. I could even kill deer with the .223, since I killed several with a .22-250. I've killed more varmints with the .22LR, when you count rats in a dump, as a teen. It's also taken woodchucks, grouse, rabbits, squirrels (lots), skunks, etc. Had a couple of .22-250s that took lots of chucks and crows, but since re-barreled my last one to .243 Win.