The problem I have found with the .17HMR for squirrel hunting is the loud report. After your first shot you're gonna have to wait a while for the other squirrels to calm down enough to come back out. I personally like to use subsonic .22LR or even .22 Short. They have more than enough power to kill a squirrel with a head shot and are quiet enough that other squirrels won't be alarmed by it.