The Nitrogen Gun is used by Citizen Level (CL) 40 players and above only, as that is required to use this gun and the others shown in the picture below.
It is so far the strongest weapon dealing damage with frost. It is followed by the Icebolt Crossbow, and Ice Cube Cannon.
It can be used in PvP (Player vs Player) Spaces Magma Core and Graveyard, it can also be used for wars in rivalry between players out of PvP games.
The Nitrogen Gun is the third weapon in the picture. The other guns are other citizen level 40 weapons.