I was looking for a temporary barrel to range test my new 3B and I have a handful of these things around (I store BBs in them). The ones I have are tapered slightly towards the closed end. I simply hacked/sanded off the rounded closed end and a bit off the open end. I jammed the tube into the existing 3B barrel (cut down and air restrictors removed) until the tube got stuck - and voila.
The fit is tighter near the base (and depending on how much you cut off the closed end) but does a reasonable job for both stock darts and my stefans. I imagine a slightly tigher fit all the way through could add a bit more range (i.e. allow the compressed air to build up a split second longer) but he slightly wider front end means its a little easier to load darts.
If you happen to have these lying around, they might be an alternative to the PETG that you usually have to buy from the pet store (at least, that's where I got mine).
Edited by ambushbug, 30 October 2005 - 11:00 PM.