contact range or nothing
The spread fans out across a huge cone, so at any real distance most of the shot sails past whatever you aimed at. Walk right up to something and the whole spread connects at once, which is heavy enough to erase most things alive at that stage of a run. From across a room you are effectively firing a single pellet, since only the center one goes exactly where you pointed.
the reload is the real cost
Nothing in the game takes longer between shots. You commit to a shot and then spend a noticeable stretch unable to do anything, which is fine against one target you can kite and awful in an event with things arriving from both sides. Build around mobility rather than more damage, because the ability to close, fire, and back off is what the gun needs.
getting one
The Arms Dealer will only sell it while he is standing in a Graveyard, and only once Skeletron is down, so the setup is a small tombstone patch near wherever he lives. It costs more than the Hardmode shotgun does despite being weaker on paper, which is odd, but it arrives much earlier in a run and that is what you are paying for. The four barrels are a nod to the 1979 horror film Phantasm.