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
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).
Step 2: Step 2: Inserting the Valve
Drop the valve through the topside and somehow get it in position like is it in the picture, it could be very tricky.
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
Seal the bottle with cap, pump air in it and check if there is some air leaking, if does, then repeat previous step.
Step 4: Step 4: Outline Medical Syringe
About medical syringe, it should fit just perfectly in the bottle, if not, then you have wrong syringe.
Insert syringe in the bottle like shown and outline syringe where is top of bottle so you could have orientation in further steps.
Step 5: Step 5: Outlining for Cutting
Put top of the cropped bottle on syringe and outline edges and middle. The middle is where you'll be cutting hole.
Step 6: Step 6: Cutting the Hole
Cut the hole like shown in picture, it can not be bigger than is diameter of bottle top.
Step 7: Step 7: Editing Bottle Top
Cut it like shown in picture, you'll see later why it's look like rectangle.
Step 8: Step 8: Molding the Bottle Top
Heat up the plastic and mold it to cover nicely syringe, that way it would be tighter when glued.
Step 9: Step 9: Hot Glue Job
Put some hot glue on syringe like it shown.
Step 10: Step 10:
Put top of the cropped bottle on syringe, add some more hot glue on edges if needed, make sure it is all tight.
Step 11: Step 11: Prepare Cap
Cut a hole in the cap to insert tube in it. Of course, diameter depends on tube you have.
Step 12: Step 12: Glue It Together
Put the tube in cap and add some hot glue to make it tight.
Step 13: Step 13: Cut the End of Syringe
Step 14: Step 14: Put Some Hot Glue on the End of Syringe
Step 15: Step 15: Put Syringe in the Bottle and Add Some More Hot Glue on Edges If Needed
Step 16: Step 16:Put All Together and Watch How Good It Looks Like
Step 17: Step 17: Making a Trigger
Mark a dot on syringe where you want a trigger and drill it.
Step 18: Step 18: Trigger
As you can see and assume hot trigger works, you need some wire or screwdriver to prevent piston from going up when you put the bottle under the pressure.
Step 19: Step 19: Cutting the Piston
Cut the piston and put some stick like in syringe like on picture, that way you make sure that a piston wont go away, except you put too much pressure in bottle. If you have some other idea how to make limiter, great , because this one can't withstand pressures above about 3-4 bars.
Step 22: And This Is What Happens When You Overload :)
That would be it, if you have some questions, post it below.
Thanks for reading :D