Converting a Gas Blow Back Rifle to use HPA Tank

A few months ago I was approached by a friend and fellow airsofter Alan. He wanted to see if I could convert his WE PDW Gas Blow Back Rifle (GBBR) to be powered by an HPA tank. He also wanted the gun to have a hi-cap magazine to extend it’s number of shots between reloading. I actually had very little experience using GBBRs but I thought it would be a pretty interesting project. After examining the gun and the firing mechanism it was apparent that the HPA tank would need to be attached to the stock gas mag, and a hi-cap magazine would need to be mated to it somehow. I realize this isn’t a new or unique idea and after completing the conversion I discovered on you can see several similar modifications on YouTube. Nonetheless, I wanted to share my implementation of a GBBR to HPA tank conversion.

I added the HPA connector by drilling and tapping 1/8 NPT hole in the bottom of the gas magazine. To have the high-cap magazine feed to the gas mag, I used an extension spring with .312″ outside diameter and .026″ wire. I cut/milled out the gas magazine BB channel to accept the feed spring. I added small cut rubber pieces to keep the spring centered and held in place. The high-cap mag is attached to the gas mag using a screw down hose clamp. Other methods that are “prettier” could be used, but the hose clamp holds the mags very securely and is easy to remove if maintenance or adjustments are needed. The feed hole on the hi-cap was drilled out to allow the feeding spring to be used. To make winding the hi-cap mag easier, Lonex Flash mags were used so just a few pulls on the winding cord and the magazine is fully wound.

The gas blow back rifle with HPA tank is very air efficient. At approximately 80 psi, the gun shoots in the low 400s FPS w/.2g BBs.

Here are some pictures of the conversions I did for Alan (WE PDW) and also for Marcus (WE L85):

Sid M. WE PDW HPA Conversion

Sid M. WE PDW HPA Conversion

Sid M. WE PDW HPA Conversion

Sid M. WE PDW HPA Conversion

Marcus F. WE L85 HPA Conversion

Marcus F. WE L85 HPA Conversion

Marcus F. WE L85 HPA Conversion

Marcus F. WE L85 HPA Conversion

Marcus F. WE L85 HPA Conversion

32 thoughts on “Converting a Gas Blow Back Rifle to use HPA Tank

  1. Hi. I have the WE PDW KAC, and I would really like to convert it into using HPA. but I don’t want to connect the hose in the mag. Is there a way to install the hosing in the grip like in so many other rifles?
    Daniel Manniche

    • Unfortunately the way gas feeds into the gun to power the nozzle and drive the bolt block is from an inlet in the magazine well and feed by a gas block inside the magazine. To have the line go through the grip would require a completely different internal design.

  2. Is there any way i can purchase one of these modified mags, but have it work on a WA/King Arms platform ? Thanks in advance.

  3. do you think one could install a polarstar v2 HPA fusion “gear” box in the WE PDW? I have one, love it, but live in a cold climate and sufffer greatly from “cool down.” I’m fine with the 30 round(i like mil sim) mags, but want to use the HPA through the hand grip. Any ideas? thanks!

    • Unfortunately the HPA input has to go to the gas block so it feeds the gun system correctly, and the gas block for the WE PDW is in each mag. So the HPA plug has to go to the mag. Sorry.

    • The polar star is a replacement gear box, since gbbr do not use a gear box it is incompatible with the we pdw. If you want to use the fusion engine then you need an aeg body.

  4. I would also like to know if there is anyone willing to make and sell one of these high cap w/HPA magazines for a WA/King Arms platform. Thanks

    • Unfortunately I no longer do the hi-cap mag with hpa gas mag modification. While it does work it can be unreliable and prone to jamming due to the exposed feed spring and the path the bbs need to take to feed. At the moment I don’t know of others that are willing to do the modification but it’s been done before so I’m sure there are folks out there. I’ll post links if I find any.

  5. I’m wondering why didn’t you just tap the mag well and put a hammer valve in the old mag hammer valve position, then run a hpa line to the new hammer valve. Then you just need to mod any aeg hi cap or mid cap magazine to fit the modded magwell. If course, you will also need to figure out how to do the bolt stop

  6. QQ, will the 1/8 npt from Redline airsoft just screw in to where the normal gas valve on the bottom of the WE gas mags unscrew? I’m not looking to make my mags hi-cap just want to use hpa for the mags.

    • Unfortunately no, the gas valve on the bottom of the mag is too small. You’ll need to drill it out using an “R” sized drill bit, and then tap it for 1/8 NPT threads. Some mags require adapters to 1/16 NPT as the 1/8 NPT may be too large of a hole to work for the mag.

  7. I have a more detailed video uploaded in the annotations of that same video showing their ingenious and simple design

  8. You think it would be possible to convert a WE g39k to use HPA. If so what parts and tools would I need. Best way to contact me is through kik SB_124

  9. Hey man! Nice write-up.

    I found this whilst looking for information regarding my own project.
    I have ordered one of the HPA plugs that goes into the magazine og my WE Scar-L. The only worry I have, is that I dont know how I am supposed to prevent leaks.

    Normally, you apply silicone oil and light pressure when the magazines are out of use. In this case, the HPA plug will not only supply dry air, but also leave the magazine un-pressurized.

    How did you bypass this problem?

    • I’m not sure pressure is needed in the magazine when no in use, but I don’t personally use GBB guns, so I could be wrong. I would recommend periodically spraying a little silicone oil into the mag and also into the gun. Since the HPA system doesn’t deliver oil automatically, it would have to be done manually.

  10. I have a budget of $300. And I want to convert my WE TECH SCAR LITE to scar has a few problems. It likes to let out a lot of gas when you shoot it. And then when your almost out it lets it all out. And it’s a really big problem so I would like to convert it. But I don’t like the HPA mags. Is there a way for it to come out of the pistol grip and the gun to take AEG mags. I’m on a big budget and I was thinking of getting it for $150 because I need a air tank air rig etc… And I also was wondering if you have some black parts for my scar to make it two tone maybe. Thank You

    • Unfortunately GBBRs are not good starting platforms for HPA engine conversion. The internals and interior geometry are just too different to make it cost effective or practical. It’s much better to start with an AEG platform. Sorry.

  11. Starting with an aeg is ok if you want to spend a lot of $$$$ on a polarstar or other
    polarstars are a good idear but if the site limit is 370fps on .2g bbs
    You may be using hier fps but you running .36g how are sites going to no your playing fair but if your like me and you want blow back and the nose and
    i want to hpa a WE apache A2 with a 370-440mm barrel with mock supresser and NPAS but am a i able to go agenst a polarstar player also if you get shot point blank with . g bb you want to thump that player persificly if thay are point blank
    If enay of you have hpa a apache A2 i woad like to hear from you if pos but if i have afended you i apologies

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