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All Springer/NP/PCP Air Gun Discussion General > Engineering- Research & Development

MAC-10 Style BB machinegun with pop-off valve - concept

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jackssmirkingrevenge:
I've experimented with pop-off valves as a way to feed a repeater for some years now and it's a sound concept.

The trigger valve allows air to enter the firing chamber which is kept shut by a spring loaded piston on a smaller diameter seat.  Once a certain pressure is reached, the piston is unseated and with its full diameter exposed, the spring is overwhelmed, allowing the air to flow out of the firing chamber until the pressure drops sufficiently for the piston to re-seat, allowing the cycle to start again as long as flow is maintained from the trigger valve.  The rate of flow effectively controls the rate of fire, while the spring pressure on the piston controls the firing pressure and therefore the power.



There are some important variables to be taken into account to make sure that the piston reciprocates as desired:

- the piston diameter must be significantly larger than the seat diameter in order to overwhelm the spring.

- the cylinder exhaust port to the barrel must be exposed the instant the piston moves, to avoid the piston "hanging" in an intermediate positon.

- the cylinder exhaust port should be significantly smaller than the seat diameter in order to allow sufficient pressure buildup to "pop" the piston.

The mechanism could be made to fit in a MAC-10 style envelope with a 13ci air tank as the grip incorporating a tube magazine with a direct blowback bolt taking care of the reloading:

Rob M:
cool.. i don t see whats powering the top hammer to cycle the BBS at first glance?? i assume is the pressure between the bb and the bolt that fires the bolt rearward on a super light spring ( like a pen spring

jackssmirkingrevenge:

--- Quote from: Rob M on February 11, 2019, 02:29:40 PM ---i assume is the pressure between the bb and the bolt that fires the bolt rearward on a super light spring ( like a pen spring )
--- End quote ---

That's the idea, since the air pressure itself opens the piston, all the bolt has to do is reload the next BB so the spring does not need to be that strong, only enough to ensure that the BB being fired has left the barrel before the magazine outlet is exposed.

jackssmirkingrevenge:
I made a little prototype based on this design with the pin valve of an HPA bottle functioning as the trigger valve:



It works so far as pulsing the air goes, but it's so fiddly in terms of sensitivity to flow that it reminded me why I had more or less abandoned this concept some years ago:

Rob M:
very nice.. any way to slow down the cyclic rate ?

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