My family had a dog that had just come out of surgery, and my mom STUPIDLY decided to remove the collar the put on the dog to keep it from getting at the stitches. Two hours later, the dog's stomach had split open, and its guts were hanging out onto the pillow it was laying on. One frantic phone call to the vet later, my dad found out that there was nothing the vets could do, so he went a cross the street and asked the neighbors to borrow a gun (don't know what kind) and put the dog out of its misery with a bullet between the eyes.
I didn't see it happen, and I didn't hear the shot; all I remember is waking up the next morning to find my mom cleaning up the blood from the bathroom floor where the dog had been attempting to rest the previous night.
Sometimes I still think about that. A shot between the eyes is probably the second most humane way (the first being UA's method) to euthanize an animal.