Anyone know what kind of gun is this in this image. I need to recreate down section of the gun.

There is nothing in the photo to denote scale, but I had something much bigger in mind than what would fit in a tank. An 88 would be a bit over 3 inches, right? I was envisioning something closer to 6 inches, from the photo.
Don <--- not an expert, having never fired anything bigger than a pellet gun
Captured from web.archive.org on 9 Nov 2012. view this capture