My experience in using Paint Ball tanks for CO2 was not great, the adapters I bought ended up developing leaks, and add in the cost of the regulator and it just wasn't that much less than the standard 5# or 10# arrangement. I finally bought a 10# used tank.
But I still was curious about using the PB tank for something portable, or maybe as a back up when my main tank runs out. I hit upon the following idea, carefully experimented, and it worked far better than I imagined. Parts are less than $10 I think (plus a pressure gauge if you use one).
The Adapter: This is just a PVC 1/2” FPT to 1.5” Pipe adapter. A 1.5” Qwik-Cap on top, with a tire stem mounted to the middle of a 3/8” hole drilled in the Qwik-Cap (the Qwik-Cap is one of those rubber caps used to close off a PVC pipe for testing or as a temporary plug). A 5/8” dowel acts as a plunger. It is cut to 3/4” long, with an upholstery tack on the pin valve end, and cross groove cut in the top to assure gas can flow (outlined in black marker in the pics). The hose is 1/4" ID, 3/8" OD hardware store vinyl hose. I put a ~ 1/2” length of 1/2” ID braided PVC hose over the tire stem/hose, so the top board could push below the level of the top, and a hose clamp over that.
Pushing the top of the Qwik-Cap, using a board with a hole for the tire stem, depresses the dowel which opens the pin valve on the PB tank. The ropes are just allow movement and keep everything together.
The Qwik-Cap presents about 1.8 square inches of surface, so as the pressure builds it is harder to push down. This provides an excellent feedback mechanism, essentially self-regulating the pressure. It's easy to depress to get up to serving pressure of ~ 10 ~ 15 PSI, but gets really hard to press above ~ 30 PSI. Depressing the pin slowly releases only a small amount of CO2 so the pressure is very controllable. If the pressure gets higher than ~ 40#, the cap will pop off and release any excess pressure, so that is an added safety factor (I tested that with compressed air).
But, this is still dealing with the high pressure side of an unregulated source of CO2 and is potentially dangerous. Like anything, use your own judgement on this.
I added a check valve and tee'd in a pressure gauge so I can monitor the pressure. With the check valve, any small leak in the adapter won't bleed down the keg. So this works similar to the little CO2 cartridge trigger guns, but really gives much more control, due to the feedback from the pressure pushing back on the Qwik-Cap. With those CO2 cartridge triggers, I think you can easily hit some very high pressures, but with this it really takes muscle to get beyond 30# (so just don't use a lot of muscle!). If you are just trying to top off for serving pressure, the pressure gauge isn't really needed, just give a little shot when the pour slows down. I used Luer connectors on this one, as that is what I'm using for my connections on my portable 6L Tap-A-Draft jugs, but use whatever matches your system.
Two more pics next...