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Picture of Simple and powerful airsoft gun using bottle and medical syringe
Everything you'll need is in this picture. I think that all is clearly visible. In some may be required drill or something to make holes, some hot nail will serve just good.

The principle is, you will use the bottle like air tank, air will be filled with pump trough valve taken out from bike tube, medical syringe will use like trigger which will be triggered by pulling out some wire or screwdriver (in my case) from piston. The bullet will be located in cap,more accurate, in pipe ( i prefer that you put some rubber on begging of tube so BB-s don't fall out) . And that the whole story, now lets go to work.

Step 1: Step 1: drilling and setting the valve

Picture of Step 1: drilling and setting the valve
Drill a hole in bottom of the bottle to fit valve inside, make sure you drill it in the middle because there is the thickest layer of plastic (for every case).