I am not sure uf anyone has ever noticed this, but I've never read about it before. On the proton gun, it is my opinion that the barrel goes through the gun box and comes out the back; it is the round part that has the green wire connecting to the box above. I have looked at all the pics from GBprops.com, and noticed that it lines up correctly. I do not know if it is an “inner tube” that has to do with the extending or not, but I think it is. Look at a pic from the side, you can see part of it is cut away so it fits nice and sqaure with the box with the red light. I believe, that the green thumb switch between the thumb box and main gun box has a wire that runs into the long rectangle box w/red light, into the bottom tube, through the main box, out the front, under the barrel grip, and connects to the tip. Look at as many pics as you can and see for yourself, tell me what you think. It makes sense and I may make mine this way. I know this may be confusing, but I'm sure w/the pics you'll see what I'm talking about.

I'm a little lost as to what you are referring to, but yes, the green lever is connected to the tip to extend it.

Yeah, it's tough to explain without a model/pic to use as reference with what I say, but I'll try to explain again.
For most of us, the gun barrel goes into the main gun box and has an open end inside with wires, etc. I believe the barrel goes THROUGH the box, and the “butt end” is the part we see that has the green wire connecting to it. It would be capped off at the end, which is why it looks like a film canister. So instead of a barrel that goes into the box, and then a seperate little cylinder at the back w/a green wire, it is the same piece that goes completely through the main gun box. I also believe it can help to explain where the possible wires from the green level goto in order to extend the tip. I could be wrong, but it seems correct so far, and I'm probably going to use this method on my gun.

I know what you're talking about but are you sure that they are both identical in width and positioning? Very interesting though.

it does not go through the body,