In the previous mission, you need to knock out that guy with the key card (Near the drone inside the gate) and not actually kill him, then in this mission when you attempt to open the locked door he will open it, then you can knock him out and go get the gun.
![Perfect Dark - Area 51 - Rescue (Special Agent) (2000) [XBLA]](http://i.ytimg.com/vi/27UwGwHOmm8/mqdefault.jpg)
How are you able to enter that locked door to the right of the first elevator to get that gun?
In the previous mission, you need to knock out that guy with the key card (Near the drone inside the gate) and not actually kill him, then in this mission when you attempt to open the locked door he will open it, then you can knock him out and go get the gun.