OK friends, here is my newest idea for the gun tip extension, one that may be the closest to the movies.
As I was walking out the door from my dorm, I turn the lock so it wouldn't lock behind me and when it clicked, something clicked in my head; Use a lock mechanism for extending the tip of the gun, here's how it could work. . .
Picture the gun grip/barrel as the “door knob” and the extending tip as the “lock” (the lock you turn on the inside, not on the outside where you put the key). In order to pop the barrel/tip out, you first turn it and “lock it”, then turn it back. The tip will be in the “unlock position”, but the mechanism inside will still be locked ready to pop back into the unlock position. In order to pop out the tip/lock, you simply turn the grip/barrel/knob and the spring pops the tip out. It all came together and so far seems to be the best idea. I won't be able to test this until the summer, but please let me know what you guys think! It may seem confusing, it's hard to explain it without a model, but goto one of your doors and play around with the lock and you'll see what I mean.

Sounds like a plan, however, do you think you could dismantle a door lock assembly to fit the barrel of the gun? This idea sounds pretty close to the spring loaded tip, but its not a bad idea

Try a spring loaded toilet paper tube holder type thing..

I have tinkered with the toilet paper roll idea, and it never worked out for me. Obviously this has to have a spring in it, which the door knob does. The prolem is that you need to trigger the spring without using a pin. If you look closely at the Gozer scene in GB, right after Peter says “make ‘em hard”, you see the guys exend the tips. Look closely (I think it may be Egon’s gun) and you do see him twist the barrel quickly back into place. When you see this, the tip does not extend up like lipstick, but rather springs out and locks into place, with the grip twisting back into place. I'm going for the solid click and lock mechanism. I have no idea if I can find a door handle set small enough to do this, we'll see. Maybe I'll goto Home Depot this weekend just to get one to play around with. (It's hard having a Home Depot across the street, 50 ft from my dorm, but not being able to build anything lol). Thanks for all the input so far!

Well, in reality, the tips were not controlled by the twist of the handle.. rather by a lever next to the trigger box.

I used the spring from a toilet roll holder. But then I realised that it was a lot more difficult to go to the toilet when there was nothing holding the toilet paper up, so i had to buy another holder, hehe.

Anyway, the spring in a toilet paper holder is very good for using in your Particle Thrower barrel. I rigged up a different system to the movie, with a button on the actual barrel and not on the trigger box (barrel doesn't twist, either). It's kinda crude, but it has never failed since I made it two years ago. Now I just gotta finish the rest of the gun!

what i dont understand is, that in some scenes the barrel is LONG and then in the next scene the barrel is SHORT.

Like in GB 2 in the Courtroom when they switch the guns on you see a SHORT barrel… then after shooting the barrel is LONG… why????

The gun tip is part of the whole filter of particles and beam extension deal. AJ, why dont you head over to the mall of america, crack open that packs case, and investigate the gun? Give us a ring when you get out of jail

Actually, I'm sure I could just pay them.. I've heard that an another MN GB asked them and they only needed $50 bucks.. Maybe..