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GBB NPAS kits.

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How do they work?


Standard GBB nozzles split the gas forward down the barrel and backwards into the recoily bits, I get that.


But once you fit an NPAS what happens?


I personally imagine that the gas sent backwards will always have to be the same amount, REGARDLESS of how open or closed the valve that lets the gas into this part of the system is.

Reason being, the gas being expelled is what causes the gas to shut off, as the recoil mech, recoiling, is what pulls the hammer back again, off the gas release button on the magazine. Yes?


Meaning that the NPAS just alters the amount of gas being sent forwards, more for high fps, less for low fps. Meaning felt recoil is the same, regardless of fps.




It could also work like this:


The gas is split to forwards and backwards into the recoil bits.


High fps = less gas being sent backwards (the amount used for the recoil parts is unrestricted, or at least it is in my imagination with regard to this theory I've invented, so by less I mean, proportionally to what is being sent the other way, otherwise upping the fps would eventually result in the gun being unable to cycle, or for it to be forever unable to cause the bolt/slide to lock back on an empty mag, regardless of remaining gas pressue) and more forwards


Low fps = more gas being sent backwards and less forwards. So theoretically, if we're going by this theory, the recoil ought to be harder if the fps is lower because a higher proportion of the gas being used upon firing is being sent backwards.


I've never had an NPAS kit, so I don't know which theory is right, but to me they do seem equally viable. Maybe it's a weird hybrid theory that's true of real life. Who knows?







Once we have that decided. What effect would CO2 have if introduced to an NPAS system? (Negative pressure adjustment system system? Yes)


As it's under a higher pressure (because it's a stronger gas), if we're going by the first theory, does that mean less of it will be needed to cycle the bolt/slide because it's of a higher pressure than green gas/propane? So per shot, it's more consistent?


Also, if the lowest fps with green gas is, say, 300fps, will that be the same with CO2? Or will the lowest setting be increased because of the higher pressure?




The basic reason for this thread is that I'm planning on one day buying the RA Tech L85 and I want to run it using CO2 with an NPAS kit. When I used to have an M16 GBBR I found that it didn't run very well with CO2, my understanding as to why, is because the buffer spring wasn't strong enough to cushion the bolt from the higher pressure, so it just felt like it was shaking itself apart and I want to be more aware of these issues before I buy another rifle, as I know the L85 can't have an enormous recoil spring like the M16 because the space inside to fit one behind the bolt is super super small.


Basically any help is appreciated, as per usual.


And also as per usual, I profusely apologise for the wall, or rather, sky scraper of text.

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I fitted an NPAS to my WE PDW, tis a valve you fit into the "gas block" before the nozzle which you open and close via an allen key.


The tighter the valve restricts the amount of gas leaving the nozzle thus lowering the fps, the excess gas that would have origionally pushed the bb forward is expelled through the "blowback engine".

This is why you hear of peoples gbb rifles destroying themselves when being run on higher pressure gas like propane in summer with an npas installed.

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So restricting the fps does increase the amount of gas used in the blowback mech then?


That's good to know, lower fps = harder kick, gotcha.

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