Part 4757K57 from McMaster-Carr is a 3-way PVC ball valve that looks to be an excellent way to solve these problems. It is made to fit 1/4" tubing, but the plastic base on each side is for all purposes the same size as 1/2" PVC pipe. Which means that (with a bit of sanding) it can fit couplers on it. With the flick of a switch, you can change barrels. Handy for when you need a shotgun blast, or for those game types where you want an arrow to hit someone with a shield.
These do cost $15 each, unfortunately, but my place of work was lucky enough to have spares that I could take. I'm looking forward to trying these out at the next Columbus war to see if the make life easier.
EDIT: Valves also exist for the 3/8" and 1/2" tube sizes, and these should work better in terms of airflow.
Edited by Buffdaddy, 27 July 2010 - 08:53 PM.