Hi Richard, I'm afraid it's the glass and there is a big chunk missing in front of the bulb. I'll try a post in the wanted section.
By the way, has anyone seen/got one one the Military Muckabouts replica GPMG, I've seen comments to the effect that they are not well finished. People seen to say that the one made by XFire is better quality but when I follow a link to their site it's a dead link. Again, anyone know if they are still trading?