AAP Carbine - HPA pressure, barrel length, and potential volume issues

[The_Lord_Poncho]

So I have a AAP based carbine - built up around the TTI PCC kit, and currently enjoying a 21cm Unicorn airsoft barrel and bucking. I've run a game with it using the rather nifty Airtac MP5 mag HPA adapter, and it performed....adequately. Aside from the outer barrel being rather wobbly affecting accuracy (nothing that a bit of threadlock can't solve), I was running at around 1.4j with 0.36 BBs with the reg set on 120PSI (i.e. green gas pressure). As the carbine is locked to semi, I would like to exploit my site's DMR rules, which allow 1.9j (i.e. equivalent to 450fps on a 0.2 BB). I don't really want to achieve that by simply turning up the regulator, and put additional stress on the gun, so I've invested in a 310mm Maple Leaf inner barrel, and I'll probably up my BB weight to 0.4g to gain max benefit from joule creep.

However, and this is the question, is the 310mm barrel going to be too long for the amount of air/volume that an AAP would produce, especially when paired with a 0.4g BB? I could presumably (and hopefully) turn my regulator down to lower pressures, e.g. 80-100PSI with the extra FPS added by the longer and marginally tighter barrel - would that therefore make the magazine valve a little softer for the AAP's hammer to strike, and thus release a touch more air to compensate for the additional volume required by the longer barrel?

 

[Sewdhull]

I would suggest you would not have issues, altho a different rocket valve might assist if you do. Lower pressure might work for you in a longer barrel as the rocket valve will be slower to seal, but I have no experience of HPA, so maybe others will chip in.

The valve on the magazine is held open while the action cycles, it's the rocket valve that allows gas to the bb, affecting gas pressure to the bb and consistency, more pressure creates more flow and depending on the design of the rocket valve that determines the pressure and gas flow .

Once the magazine valve is open the pressure is released as far as the valve is concerned ( it's similar both sides of the valve) so I don't think pressure affects the valve opening, but will affect the flow rate, esp HPA.

With hpa you have as much gas as you want so you can get lots of joule creep with longer barrels, with propane etc you are reliant on the liquid boiling off to maintain pressure as the gas is used so you can run out of gas in a shot, but ready to go for the next.

Great description of GBB operation follows,

 

[Krisz]

I don't think you can have volume issues on HPA simply because you're gonna have the same amount of air with each trigger pull.

HPA is the most consistent system at the moment.

It's seems that you found out your gun's limit within green gas pressure PSI.

 

[Sewdhull]

The benefit of HPA is that the pressure doesn't drop (marginal drop) as gas is used in a shot and has less cooling ( no phase change). Both of these things create better consistency.

If the pressure remains constant vs propane for example where the gas gets used up and pressure reduces in a shot, you will have more gas per shot for HPA. In a gas pistol, the rocket valve will limit the gas to the bb (its not really regulating it), then the rest of the gas operates the BBU.

My understanding is that the rocket valve will close when the pressure behind is rises high enough, that this happens when the bb leaves the barrel and the pressure on the forward side of the rocket valve is released so it is pushed closed and if there is enough gas left over cycles the pistol for the next shot.

If the barrel is longer this will use more gas, and get more energy to the bb. With propane the amount of gas per shot is limited ( the pressure takes time to be raised again within the magazine) but with HPA you aren't limited by gas volume and can extend your barrel and get more energy into the bb.

 

[The_Lord_Poncho]

Top stuff, thank you both! Sounds like I'm not going to have an issue with volume then when I fit the longer barrel (and I've learnt something too to boot). Ultimately the test will be when i fit the barrel  - it turned up yesterday, so I'll get it sorted this weekend and report back!