1. Running the rifle with a remote line to a big bottle is undoubtably the cheapest...but really limits mobility. the one below is running on a 20oz. bottle through a remote line (although you could use other size bottles, the "rest" it is being shot from was made to hold a 20oz.)...no problems shooting from a bench, but wandering the woods with that bottle and line isn't a lot of fun.
2. Running the rifle from an attanced larger tank is nearly as good, but does add weight and requires specailized adaptors for the QB78. the one below is a QB79 Iat least in US marketing) and was made to run on attached paintball tanks. Will run on the little 3 1/2oz. tank show here, or a 9 oz. tank. Larger tanks are too big in diameter to fit.
Bulk filling needs an bulk fill end cap...has a quick detach fitting and what is basicly a one-way valve: gas can go in, but not escape back out. Filled from a larger bottle, the typical QB tube will hold about 2oz. of CO2 (although I'd limit it to 1 1/4 to 1 1/2oz. on warm days)....not wise to completely fill the tube with liquid co2, something in the 70% full range would be a real max...and something closer to 60% of the volume filled in warmish weather. Not as easy as it sounds, but you'll learn the tricks soon enough. Couldn't find an individual picture of the QB78, but the one below is set up for bulk fill.
While I know the UK has paintballers...so i assume they have paintball shops that fill paintball co2 bottles. These bottles (used in #1 and #2 above) are the most common way to bulk fill. I'll post directions to a tutorial bulk-fill site if you ae interrested...as mentioned, it's a little more complicated than it seems (but not difficult).
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CO2 consumption has stayed about the same as when using the 12gr. cartridges. If your rilfe got 40 good shots from two cartridges (which is low...that would be about right for one tuned to about 600fps)...that would be 40shots for 24gr....or about .6gr. per shot. That's going to be close to your consumption no matter what size the tank. So figure 28gr. per ounce, or about 46-47 shots per ounce. So a 4oz. bottle should work out to something close to 188shots....and a 20 oz. bottle about 960.
Using US prices, that 20oz. paintball tank (which costs about $4-$5 to fill) gives as many shots as 48 12gr. cartridges ( which at the cheapest, is going to cost $19-$20).
But besides cost, ther rifles have all shot better attached to big tanks than they did with 12gr. cartridges...just a bigger volume/larger heat sink, which keeps shots more consistant.
