When doing at home/emergency euthanasia, the goal is to destroy the brain. You can use a shotgun, a hammer, a screwdriver, an icepick, etc. Unless you're competent in the process, causing broad range damage is your best bet at getting the job done without causing additional pain. I use a pocketknife because I've always got one on me and when I have something that needs to be put down, waiting around isn't doing either of us any favors.