Hunter's Handgun is a ranged weapon added by Xeno's Reliquary. It uses different kinds of bullets loaded in a magazine as ammo. Holding down right mouse button with an empty gun will load it if there is a full magazine in inventory. The color of the grip indicates what type of ammunition it is loaded with.
If there are multiple types of magazines in the inventory, the Hunter's Handgun will load the one in the first hotbar slot. Note that when spawned in creative mode or via NEI's cheat mode the durability bar will remain full at all times. It will display the amount of shots left in the magazine if crafted.
The Hunter's Handgun can be crafted with the three separate gun parts (found in top-tier dungeon chests), a slimeball, and four iron ingots.
The complete Hunter's Handgun