Nailgun

The Nailgun is a Beta weapon found in Grand Theft Auto: Vice City and Grand Theft Auto IV.

Grand Theft Auto: Vice City
Not much is known about the weapon but it was most likely a weaker version of the Pistol featured in the game. Its model (nailgun.dff) and its textures (nailgun.txd), still remain on the PS2 disc. The Nailgun can also be seen in Tooled Up but it can't be purchased there.

Grand Theft Auto IV
There is a fully functional model in the game files and the weapon works like a machine gun. To use the Nailgun, the player can replace the pistol with it by editing the game files. It functions similarly to a machine gun when used.

Trivia

 * The is featured as a weapon in Manhunt, another video game released by Rockstar Games in 2003.