Yeah, I'd try buying a valve locally, your friend might be able to get you what you need. If not, some of the smaller locally owned farm type stores seem to have full port ball valves gathering dust.
I've mostly dealt with smoothbores, here's an idea of what my smoothbore .40 QEV airgun can do offhand at 20 yards using patched 3/8" ball bearings:
Not a tack driver, but plenty good enough to hunt.
For a practical semi auto, you might look to pick up where I left off on my balanced spool valve. This winter I plan to take the project back up myself. The other way is to use a 3 way valve to pilot a QEV.
Staying subsonic is very important of course for noise, as is using just enough air to propel the projectile to the desired velocity, and no more than that. Since supersonic speeds are difficult to achieve with airguns, this is the biggest factor in reducing noise. Helps a lot with accuracy too. You'll find that higher pressures are much more efficient than lower ones. The 400 to 600 psi a fridge or air conditioner compressor can achieve will give plenty of bang for the buck.
That shooting shack sounds like a good idea, might have to try that one myself.
