I mentioned that I was building an airgun... Here she is!   This is pretty crude, mostly a test-bed.  Still, pretty darned effective.  I've been shooting 1/2" marbles out of the thing and they come out with authority; they will penetrate several sheets of drywall.

Pics of the mechanism:


And the whole gun:


It's a simple ball-valve, inline with an air chamber made of 1 1/4" pipe with a schraeder valve in one end.   1/2" copper tubing for the barrel.   
I pressurize to 125 psi with a bike pump, only takes a few strokes.
I plan a more complex reciprocating-valve model but I need to get some plumbing parts.

How accurate is it?  And how far will it shoot a marble?  I think it is great, it reminds me of some of the 18th century muskets that used compressed air.  Bill

I do have a nifty little video camera and might give it a try.     Haven't got much of a facility in my back yard for shooting....

The air chamber is just a standard cast-iron pipe nipple....No idea what it's pressure limits are but likely rather high.   Guys working with higher pressures usually use things that are both stronger and lighter.
Mine could probably use a bit more volume, but it does seem to work well.

The copper tubing barrel is in fact nominally .50 caliber, that is the inside diameter.
I was lucky to find some 1/2" marbles... Standard ones are a bit large.   These I found in a goodwill for a buck for the bag...
Other projectiles include darts (made from nails) and AA batteries....

Mine is a muzzle loader but it's pretty easy to make a breech of sorts; you just cut a port in the barrel and insert a piece of close-fitting tubing with a bolt threaded in.
There are several of these up on YouTube.   Some guys have even added laser or telescopic sights.

OK I am very impressed. Now go to a gun forum and try to get a reg. stock for it. Since you found the way to build it now just tweek it and have fun!

You might try a large (homemade) Schrader valve next time.  That's what the Girandoni used.  It'll provide multiple shots quite efficiently.

If that's a standard 3/4" galvanised pipe you're using for the chamber, it should (DO NOT QUOTE ME ON THIS, I CAN'T FIND MY CHART) hold something like 2,500 psi before failing, BUT you should only fill it to 1/3 of that for safety reasons (1/3 is the standard hpa airgun margin for error).  Please recheck this before you do it, I don't want a fellow slinger filled with shrapnel on my account.

You'd blow out your bike pump way before you reached 1000 psi.  I speak from experience...