I had to do it once as an officer. The animal was a wild boar that had made its way into a residential area. I went after it carrying my Mossberg 500, loaded with 00-buck. I followed it for a few blocks on foot, until it dove into a waterway between two streets, and behind a home at which a backyard BBQ was taking place. The family had a boat docked back there, and the father, a veterinarian himself, took me out on it to catch up to the animal. Once we pulled up on it (hogs are remarkable swimmers!), I decided I was too close to use the shotgun, and set it aside for my .38 revolver, loaded with Winchester +P 95-grain Silvertip hollow-points.
My first shot struck behind the nape of the neck, slightly off to one side, and lodged in fatty tissue near the shoulder. The hog dove beneath the boat, and re-emerged on the other side. My second shot struck along the rib cage on one side and had little, if any, effect. My third struck right behind the skull, shattering the spine, and causing the animal's head to go beneath the water. The animal lived just long enough to drown, judging by the volume of water we later drained from its lungs. He was too heavy to pull aboard, so we towed the carcass to a nearby boat ramp.
Got quite a bit of attention there from the other boaters coming in from their Sunday fishing trips!