The real advantage of the shotshells may be their safety. One is less likely to shoot through the hull of a boat with them than a real bullet. Many halibut fishers prefer a single shot, stainless .410 "Snake Charmer" shotgun for its simplicity and with greater length, it is still easy to operate one handed over the side of a bigger boat for the dispatch and keeps you dry. Shotguns are legal/less hassle to bring into Canada too versus a handgun.
Now we harpoon them. The key is to not lift the fishes head out of the water as you reel it in as it approaches the boat. They stay calmer and provide a steadier target for the harpooner. A quick thrust and the rope with a buoy attached follows the big fish when it swims/dives in its violent death throes. The buoy resurfaces like Quinn's shark in "Jaws" and you pull it in.
Sorry for the hijack, but it is another way to have fun with a Smith and Wesson.
Clean Break