After converting a Umarex Air Javelin arrow carbine to use either 13ci HPA tanks or 88 gram CO2 cartridges recently, I decided to remove the butt stock and cut down the barrel by a few inches to make it onto a large arrow pistol. The carbine is 32 to 35 inches long (with the stock extended) and has a 12 ½ inch barrel. A shortened pistol version which can use a 13ci HPA tank or the shorter CO2 cartridge can be cut down to only 19 inches long overall with a 10 inch barrel. And an even shorter 16 inch pistol version with a 7 inch barrel could be made if you only use the 88 gram CO2 cartridges. In each conversion, the barrel shroud ends up about ¼ inch beyond the front edge of the HPA tank or the CO2 cartridge. I've attached some pictures which should explain things clearly enough.
A Javelin pistol wouldn't be much good for anything if shortening the barrel reduced the arrow velocity too much compared to the original 12.5 inch carbine barrel. The velocity and FPE results for CO2 and for 1100 psi air in the three barrel lengths was interesting and appears in a short table at the end.
There are two main steps to converting the carbine to an arrow pistol.
Step 1: I began by cutting off the rear of the butt stock just, about ½ inch behind the point where the narrowest section of the stock connects to the rear of the gun. There is an internal partition inside the stock at that point. The picture below shows how it looks after the hollow stock has been cut away and the outer edges filed down to expose an internal partition. The small hole in it is for the hammer spring preload adjuster.
There are good videos on Youtube showing a disassembled Javelin and where the adjuster is located within the stock. If you cut off the stock any closer to the action the hammer spring will fall out. Safer to cut about 1/2 inch back from that internal partition so you can see what you are doing, and then used a fine tooth saw and file or sander to finish up.
Step 2: Use the same general approach to cut down the shroud and barrel for either length of pistol. If you convert your Javelin to use a 13ci HPA tank, you will end up with a 19 inch pistol with a 10 inch barrel. This CO2 to HPA mod is described in an earlier post from an hour ago on this same forum;
And you could still use CO2 in the same gun with a 10 inch barrel;
And if you only plan to use the 88 gram CO2 cartridges and want the shortest possible, the Javelin could be cut down to a 16 inch pistol with a 7 inch barrel.
Cutting the shroud and barrel isn't very difficult if you use a cut-off blade in a rotary saw and follow up with a hand file and sander. After wrapping the shroud in masking tape to prevent scratches and to indicate the edge of the cut around the shroud, use a thin carbide cut-off blade in a rotary saw to cut the shroud but not the air tube within. Then pack foam or something similar firmly around the metal air tube within the shroud to secure it from moving around before carefully cutting it, a 5 second job. Finish up with a hand file to be sure that there are no rough edges on the end of the air tube to slow an arrow. After that, use a flat or belt sander to carefully clean up the end of the shroud.
The velocity and power summary comparing the original Javelin carbine with a 12.5” barrel to the shorter 10” and 7 inch barrels using either CO2 or 1100psi HPA air is interesting. These tests only used 170 grain arrows (120 grain shafts + 50 grain target points) and the average CO2 velocities tend to vary by day and within each shot string. I used an FX radar chrony and a Caldwell G2 light chronograph to check the velocities on the same day using about 5 shots/test.
12.5” barrel – CO2: 286 f/s – 30.9 fpe
12.5” barrel – Air 1100 psi: 278 f/s – 29.2 fpe
10” barrel – CO2: 284 f/s – 30.4 fpe
10” barrel – Air 1100 psi: 267 f/s – 26.8 fpe
7” barrel – CO2: 269 f/s – 27.3 fpe
I was surprised to find that 1100psi HPA produced slightly less power from the longer 10 inch barrel (26.8fpe) than CO2 produced through the shorter 7” barrel (27.3fpe). The power produced by CO2 usually start to drops relatively quickly after first 6 to 8 shots though as the tank cools down and empties. Arrow velocities on HPA air will remain relatively constant until the tank's internal pressure falls below the regulator pressure.
Is a pistol mod like this worth the time involved? If you have 2 Air Javelins, it's fun to have both a carbine and a pistol to compare. And it isn't a very difficult mod, just some saw and sander work. At the end of the attachments is a size comparison pictures of an unmodified carbine next to a longer 22" pistol, made by cutting off the butt stock but leaving the barrel at the original 12.5 inch length. Personally I prefer the 19 inch version with a 10 inch barrel because it's easy to handle and can use either HPA or CO2.
I've posted quite a few short and simple project summaries like this on different forums over the years as my interests have changed. And almost every week I discover similar project posts on other forums or on Youtube which open up new ways to enjoy my hobbies. What surprises me the most is that our posts on internet forums might still show up in narrow topic searches generations from now, to either help them, or possibly to just confuse whoever reads them.