A few more details on what exactly you want would be helpful

For an FPS, you could do something to this effect:

If 1 is pressed, the view entity of a pistol is turned on and the view entity of all other weapons is turned off. A variable (call it "gunMode" or something) would be set to 1, which would, in the shooting code, allow the player to shoot as if he has a pistol.

If 2 is pressed, the view entity of a sub-machine gun is turned on and the view entity of all other weapons is turned off. GunMode is set to 2 and allows the player to shoot as if he has an SMG

Etc, etc.