I know from personal experiance that a .22LR round can easily kill in one shot.
I've witnessed, and shot, deer and coyotes with a .22 rifle and dropped or had die within short distances. Head shots are always a drop in place.
The butcher who came out and slaughtered a steer for us used a .22 at 5 paces, instantly turned those lights out. Later on he slaughtered a pig for us, but he used a .223.
"Why?" I asked him out of curiosity.
"A pigs head is harder to penetrate. You want to be sure on the first shot." Was his answer. Made sense to me.
It's plenty lethal. And a .22 rifle is my personal 1st choice for survival.