I made a simple gun game, with a camera parented to a gun, which has movement.
When I go to camera view and press P, however, the screen turns grey and there are no objects. If I view from any angle other than the camera, the game works fine. I’d think if this were a problem with my graphics card, I wouldn’t be able to see the game at all.
I haven’t looked at your file, but the image shows me that your camera is parented to the gun.
You should apply scale to the gun to make it (1,1,1). Otherwise you get side-effects at the children (like looking up into the air ;)).