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
Step 2: Step 2: Inserting the valve
After you did it, take hot glue gun to seal the valve in place an prevent air to bypass a valve. My method was that I push the valve a little bit inside of bottle and tilt it on the side, and then I took hot glue and drop it from topside near the hole, then quickly take a valve and pull it down (simultaneously rotating to be sure that glue covered all edges of hole). After that I just put some more hot glue from outside to make seal it tight.
Step 3: Step 3: bottle test
Step 4: Step 4: outline medical syringe
Insert syringe in the bottle like shown and outline syringe where is top of bottle so you could have orientation in further steps.