I think it would be a better idea to discuss it in this thread not via PMs... at least others can contribute and find some useful links
here is a short .doc file explaining how directionally control valves work (the same I already sent you)
and it has been discussed thoroughly here
a .177cal semi auto is feasible if you have one of the following:
-access to 5-10fridge compressors, a scuba or other compressed air tank rated 3000 or more psi(just in case!!!) and the tools and fittings to connect these to a large unit for HPA(800psi is realistic)
-access to a large amount of highly compressed gasses(air or N2 are the two feasible ones).
one fridge compressor would be enough for a .177 or 6mm semi and full auto too... you would only need to add a tank to it with at least 5L capacity..
of course since fridge compressors can generate pressures of up to 500 psi and more you'll need a tank that can handle that...
mine is built from a lenght of malleable iron pipe... an old co2 extinguisher, co2 or HPA tank would work too