A rifle being "underpowered for hunting" depends entirely on the hunter. There was an old lady of legend up in northern Quebec who used to get a moose ever year with a .22 rimfire.
My grandfather (born 1879) killed a deer with a .22 rifle back in the 1890s, and that was when the .22 rimfire was still loaded with black powder. I certainly wouldn't try that even if it was legal, which it isn't in any state I know of.
I have read that there are many Eskimo and Aleut hunters in the far north who regularly take polar bear, walrus, caribou, and other large animals with puny little rounds like the .22 Hornet or something similar. Their secret is to get in real close and shoot for the brain. Getting in real close to a polar bear armed with a rusty old .22 Hornet would take a lot more guts than I have.