QB79 tanks...

[ribbonstone]

..just a quick note.  If you can find (ebay, swap shop, etc.) one of the old 7oz. CO2 tanks, buy it. Same length as a modern 9oz. tank, but 1/8" smaller in diameter.  Really cures the barrel-trouching problem some QB79's have.
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Small co2 tanks, under 2" in diameter and under 2 feet in lenght, don't have to be re-hydro tested.  You might have to rebuild the pin valve or at worst have it replaced, and the 7oz. tanks leaves a nice amount of space between itself and the barrel.

[wvkeith]

I cured mine with a 13ci wevo tank.

[ribbonstone]

If you want to run air, that's probably the best possible choice.  Some of us want to run at least some of our rifles on co2.