Having no clue of air guns what so ever, history or otherwise at @ age 8 or 9, built my first PCP simply having an idea and cobbling a very simple one together. My father was a previous WWII Aircraft mechanic and had lots of Surplus Hydro & Pneumatic valve fittings, tubing etc ...
An @ 6" diameter x 18" long Yellow Military ( US Air force ) breathing oxygen surge cylinder taken from some WW2 or Korean war era aircraft.
Threaded on top a needle valve attached to @ 3 to 4 feet of reinforced rubber hose ( Likely fuel line ? )
Hose terminated into a push button air squirt nosile as one would use to blow off parts in the shop.
Attached to spray outlet an @ 3 foot piece of SS seamless HP tubing of 1/4" od & inner bore that a Daisy BB would just roll down. As I recall I had placed a small pliers pinch to stop the BB near the tubes attachment point.
With a hand pump or a trip to local gas station would fill cylinder to 50-60psi IIRC.
* Crack the needle valve just far enough to create a small leak it would charge the hose to tank pressure.
Pressing the button on sprayer would discharge a small blast of air. Effectively a plenum dump ( Hose being plenum )
BB would fly out pretty hard as I recall.
It worked no lie ... sighting down barrel was able to hit a few birds as well just fool around with my first BB gun being dad had not allowed one at this point & figured this contraption his son had made was so low powered it was just overlooked as being serious.
From there ONLY RECALL GETTING IN MUCH TROUBLE !!!! BB over the back fence striking a neighbors Back porch sliding glass window and taking it to pieces of glass shards. ( Pre safety glass era being we're talking @ 1967-68 there about )
So I had freely conceived a PCP BB rifle never having a clue of such a devise .. just did it cause I could.
TRUE STORY
