Not much is known about the Rubber Gun's specifications. Except that, as seen on the image, it is based on the Israeli IMI Uzi, similarly to the Gold SMG from The Ballad of Gay Tony, although with a blue camouflage. Based on its name, it is apparently a toy gun able to shoot foam/rubber pellets. However, it is unclear if the Rubber Gun is actually a toy or a disguised weapon (like the GTA III Micro SMG).
A cut firing sound (ptl_rubber) exists in the game files. The file's prefix ptl indicates it would have been in the Pistols category, even though its design is closer to that of a submachine gun. Other in-game filenames however suggest that it was a submachine gun instead.