Am pretty sure there must be some fault with this inelegant solution, but it did work. Did own an “F” and did something very much like this with it (but it was a later “F” with the inside threads).
This is for a Challenger 2000 co2 rifle...pretty much the same tube, vent hole, but made to run real slow.
Cut off a chunk of some cheap Mag-Lite wanna-be flashlight (think it was a “gimmie” light with some company's logo stamped on it). Decent fit to the inside of the tube.
Cut two o-ring seats and let it straddle the vent hole. Didn't seem inclined to move, so was just a friction fit (pressure on both ends of it, a tight fit, so it just stayed where it was). Think of how the Discovery mounts the gauge in the tube.
Standard bulk fill end cap.
Didn't leak...and on a gun already designed to get 70-85 shots on a 12gr., seemed to shoot forever. Couldn't think of a real good reason to keep it as bulk fill, so just considered it an experiment to see if it could be done.
So let me have it Tim....must be some problem, even though it did work...just seems too simple. Had considered tapping that spacer for a 1.8K burst disk, both as a safety and as a way to keep the spacer from moving...but gave up on bulk fill before going that far.
As to how to know how much to fill....would need to estimate the volume of the gas tube in either cubic inches or cc's. IF you use the normal ratio of gas to air that is used in paintball tanks, then soemthing like 1 ounce of co2 per 43cc's.
How to know how much is in the gun would require a scale. Just get the weight of the gun empty...then weight filled. Most digital food scales can handle the job, at least to the accuracy needed for this job.