It's a while since this thread started and I've been doing some thinking and fiddling in my shed.
I've turned a short adaptor from brass which holds a Co2 screw in bulb. I've kept it fairly simple and purposely made the valve a fair bit smaller than the original Giffard one, I'm more interested in seeing if it's feasible rather than producing lots of power.
I haven't tried it with a projectile (other than a cotton bud poked in from the muzzle) yet, but it certainly releases a satisfactory sounding crack when the hammer falls. I've still got some work to do this afternoon with gaskets / O rings , those 16 gr. cylinders soon empty themselves if the seals are anything other than perfect. I know that some people are interested in similar projects and I will update when I've tied it properly.