If you're still on the hunt for a barrel blank, you can get 24" barrels from Crosman pretty cheap...hit and miss quality though.
1760-001 is a 24" in .177
2260-003 is a 24" in .22
Haven't tried these and don't know anyone who has put a pellet through one as of yet, but IF they actually are LW barrels, one heck of a bargain...my last LW blank cost me close to $200. Being as you're in the US, should be no problem for you to order.

http://www.airgunsofarizona.com/FXBarrelPromo.html
If you're intending to use a rifled barrel and factory ammo, the issue remaining is consistency in releasing the air. The qev will be reasonable at that, but it's opening speed is directly related to it's venting...consistent venting is going to be a challenge.
If you can machine a qev, perhaps you can machine a different valve as well?
A QDV (search the forum) activated by a spring loaded striker would be consistent, potentially BRUTALLY powerful and fairly easy to make. Could be fitted with a sear and trigger. Also have a look for a toolies piston and btb ballanced push button valve.