a shotgun, with everything that implies
One trigger pull spends a single round and throws a fan of pellets. Point blank, most of a pre-Hardmode enemy disappears in a couple of shots and you have paid almost nothing in ammo for it. At range the fan opens up and most of the pellets sail past, so the same weapon feels broken or useless depending entirely on where you are standing.
when to pick it up
It comes out of the Underground Jungle, either from the vine covered chests down there or from crates you fish up in jungle water, and it turns up early enough to carry you through the first bosses. Fight in a corridor, let things come to you, and do not try to snipe with it.
name
Boomstick is what Ash calls his sawed off shotgun in Army of Darkness, and the sprite is drawn as exactly that kind of short double barrel.