Been searching, and have seen a lot of posts with people suggesting to others to either force a 3/8" pipe nipple into the end of the bottle or use a drill and pipe tap to tap the end of the cylinder.
There's an easier way, although I've yet to try it.
The internal threads on a paintball tank are 5/8"-18tpi (unified fine threads). These are the threads exposed after removing the pin valve.
Now, you may be at a loss for how to convert these threads to NPT threads, but take a look at this:
It's a simple 45 degree SAE flare adapter, for 3/8" OD tubing to 1/4" NPT male. Less than 2 bucks, and should be a fairly common part. The flared side is 5/8"-18tpi UNF.
Problems:
Thread engagement is shorter than the pin valve used, so I would personally limit pressures to somewhere mid-range, around 400-500 PSI max.
Orifice through the fitting is rather small so it would be best suited as an air reservoir.
That's about all I wanted to say. Rather limited application, but slightly useful nonetheless.
Paintball Tank to NPT for Medium Pressure Reservoirs
- clemsonguy1125
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Thanks, was thinking about the best way to do this earlier today.
Paintball ASA adapters usually have 1/8" NPT threads already built into them. If one doesn't mind the restriction, that is also a viable option, while not having to worry about removing the pin valve from the tank.
- mark.f
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Very true. I guess it's just whether or not you have that style adapter available to you conveniently or whether or not you want a permanent connection (can't be taken off) or not. Keeping the pin valve (obviously) lets you remove the tank from your gun while pressurized.
Unless you build a hammer valve into the other end of your 3/8" flare fitting, in which case it would be like a Girandoni or Condor system.
Albeit limited to 500 PSI until you find a better connection (longer thread engagement).
Unless you build a hammer valve into the other end of your 3/8" flare fitting, in which case it would be like a Girandoni or Condor system.

just simply use any Asa they convert to 1/8npt. flow shouldn't be a big problem if your just filling your gun. http://www.ansgear.com/Paintball_ASA_Sale_s/808.htm
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3/8 flare fitting fits in the threads on the bottle it's self that is if u can get the valve off of it
most big random like i ever seen