If you run out of bullets after taking the gun of the first armed security guard in Unforseen Consequences, the gun is holsterd. Now if you go pick up the 2 clips from the closet where you get the hazard suit the gun dosent reapear, but you can shoot. So you are in fact shooting an invisible gun.
Now if you pick up a flare the gun reapears 
Awesome remake thank you so much!!!