The Empty Gun is a weapon item sold by Bratty and Catty, who reside in the right alleyway at MTT Resort. Its defensive counterpart is the Cowboy Hat. According to Bratty and Catty, it was found (along with their other wares) in Waterfall at the garbage dump, explaining its empty barrel.
When attacking with this item, four bars appear instead of one (or a mash indication like the Tough Glove.) Its attack animation is initially a big star, which explodes into a series of "gunshot" circles with eight stars spinning counterclockwise outside of the circle.
Compared to other weapons, the Empty Gun requires more precision to use, as it deals less damage if the four bars are not hit in the center.