Why wouldn't you simply use the same air source for both guns?
Mounting a Havok launcher on a gun in a similar fasion uses the same air system to operate the gun and the launcher so it could be fesable to accomplish the same fith you FS idea.
Although I believe accuracy would suffer from the mounting possition. Harder to aim the lower gun.
But a Phantom would be pure easy to do.
Remove the trigger frame and build a simple lever to release the sear...or a button for that matter.
Use a valve body made for the bottom line...or VSC style Phantoms.
Rail mounts could be added to the valve body and the breech fairly easy.
Cut FS fin slots into a stock class breech and mount it a bit angled left or right depending on user prefference.
6" or 8" barrel would keep it short and you would simply use a micro pump handle.
Cost could be kept down to the $200 range I would think and overall length would be in the 12" to 15" area.
It would be a one shot and reload thing but that is why you have a second gun...right?
This post has been edited by Krazy8: 19 August 2010 - 08:33 AM