Earlier this year I bought three Umarex Air Javelin CO2 arrow carbines, one to experiment with mods and the other two for short range target practice. They are inexpensive, relatively low power ~30fpe and accurate enough to Robin-hood arrows at 10 yards if you keep using the same aim point. Like others, I wanted to convert them use a refillable and regulated 13ci HPA air tank instead of the disposable 88gm CO2 cartridges. There isn't quite enough space to fit a 2” dia air tank under the barrel shroud because the air tank bumps into a short portion of the narrow hollow accessory rail which hangs down about ½ inch from the bottom of the shroud tube.
There are several very good YouTube videos showing the internals of the Javelin in detail and discussing ways to convert them to HPA. But they were seemed unnecessarily complicated and bulky.
The easy 1 hour DIY way to modify a Javelin to accept a 13ci HPA tank with a regular air-to-CO2 fitting is to carefully trim off a few inches of the 1/2” wide hollow accessory rail which hangs down below the shroud. Since the accessory rail was added after the shroud tube was made, trimming off a few inches of the rail just exposes the original curved bottom of the shroud itself. It's a very simple modification which can be done with a hand saw and hand file in less than an hour. Any cut marks remaining on the bottom of the shroud are covered up by the HPA tank.
Be sure to carefully mask tape all of the surrounding areas to prevent scratches before starting, and secure the Javelin from wiggling around before you start cutting.
I first tried cutting the shroud material on the test Javelin with a fine tooth hacksaw. It worked but not nearly as easily as I'd expected. The blade bent and made a rough cut which was difficult to file smooth afterward. Then I tried using a table top band saw with ½” band. That was much easier than the hack saw, but the band also twisted and wandered to give slightly wavy cut edges which didn't look good and were were difficult to file down.
My third attempt used a small hand saw I'd picked up out of curiosity at local hardware last year but never used for anything until now. It worked just like it was made for the job. An extremely thin and flexible 2” wide push-pull style handsaw called the Vaughan BS150D Bear Saw extra fine 5.5” mini for about $15. It gave thin clean and easy to control straight cuts due to the very small and extremely needle sharp teeth which cut in both directions. A hacksaw and a band saw worked OK if that's what you have, just cover any unsightly rough scratches with a wide piece of Velcro tape as shown below. I'd get that hand saw though, you'll find other uses for it when you're done with this.
Rough spots can be followed up with a hand file. Saw marks aren't really visible afterward anyway since the two small sections of bottom rail being removed are only about ½ wide, so the HPA tank covers any narrow marks under the shroud. I put a piece of black Velcro tape about an inch wide along the narrow cut area on the first Javelin I modified with the hacksaw to smooth over the rough cut edges.
That's almost it. With a few inches of that unnecessary bottom rail trimmed off, an HPA air tank (13ci 3000psi or 15ci 4500psi) with an air-to-CO2 adapter will fit perfectly under the barrel shroud. A Javelin modified this way can use either CO2 or a regulated HPA air tank with an air-to-CO2 adapter.
But after testing it turned out that the simplest version of this modification require the use of a newer HPA regulators with a rotating collar to allow the tank to be screwed into the Javelin without the pressure gauge bumping into the shroud. In the end I bought a couple of new HPA tanks with Air Ninja Air Pro V2 and First Strike FS Hero 2.0 regulators with rotating collars in them to compare. Both work fine, the Hero 2.0 comes with the 15ci 4500psi tank.
Why a rotating collar? The older regulators require an additional hole about an inch square to be cut in the base of the shroud near the gun's input port so the gauge on the air tank could rotate into place without hitting the shroud. The bigger problem is that the pressure gauge and fill nipple might end up facing the wrong way when tightened into place. Like mine did, on 2 different tanks with different regulators.
So I switched to HPA tanks using the newer Air Ninja Air Pro V2 and First Strike FS Hero 2.0 regulators. Each model uses a different type of rotating collar which holds the tank's pressure gauge and fill nipple. An EMPTY tank with a rotating regulator collar can be screwed tightly into the Javelin while the collar with the gauge and fill port remain stationary and in a comfortable position on the gun. Once the tank is secured the regulator collar is locked into place. This CO2 to HPA modification only works well if you use an HPA tank with a regulator with a rotating (~$65) collar. That way you only need to trim off about 3 inches of the unnecessary bottom rail to convert your Javelin to air. After that you never need to remove the HPA tank from the gun to refill it. Though you could swap out the HPA tank with an 88gm CO2 cartridge at any time.
As a side project I removed the butt stock from one of the Javelins to make a sort of large arrow pistol about 22” long, using a 13ci HPA tank and original 12.5 inch barrel length. I'll post another summary shortly showing how I cut down that Javelin into a 19 inch pistol with a 10 inch barrel.
As others have already noted, the Air Javelin seems to be optimized for CO2, which is in the 1100psi range on a warm day. A conversion to HPA air at similar pressure will not produce substantially more power than you were already getting from the first few shots on CO2. Since the HPA tanks are regulated, the average shot velocity should be relatively constant until the pressure in the air tank falls below the regulator setting. Here are some average fpe levels for five 170 grain arrows (120 grain shaft + 50 grain target point) for CO2 compared to 3 different model HPA regulators set at different regulator pressures on different air tanks.
CO2 75F: 275/28.6 fpe
800 psi: 240/21.8 fpe
900 psi: 259/25.3 fpe
1050 psi: 272/27.9 fpe
Arrows tested with 75, 100 and 150gn target points showed no significant increase in fpe compared to the 50gn points, and all of the heavier arrows had velocities under 250f/s. As a final test I cut 2” off the end of the barrel tube on the test Javelin, bringing the barrel even with the flat end of the HPA tank. It only reduced the FPE by about 10% but it didn't make the Javelin feel much smaller and it still required the use of the the regular length arrows.
I included a picture of the 2 HPA converted Javelins side-by-side, along with 3 of the 6 Robinhood arrows accidentally shot during chronograph testing.