Have to mention the potential problems.
#1. Running too high a pressure. The way a QB is built, there is a great big steel ring inside the tube that the valve butts up against. This steel ring takes the stock bolt from below and the breech screw from above...two pretty beefy screws...so it's not going to move. Looks like the weakest spot in the system is how close to the end of the tube the screws that hold the gas block are. Stick to co2 type output pressures.
#2. The standard valve stem will last for awhile but will sooner or later fail (which is true of co2 as well). May as well replace it with a Delrin one and even better if you modify it to do away with the useless 12gr. piercing stem.
#3. Air is "thinner", which is why 850PSi of air gives more speed than 850PSI of Co2, but that thin quaility also promotes leaking. Co2 tends to swell 0-rings a bit, air does not. Isn't uncommon to have to have to figure out how to cure leaks.
Not all air tubes are exactly the same internal diameter, and on my 4 QB conversions, two tubes just refused to seal at the gas block. Both turned out to be a little (like .003-.004") larger. Haven't found an easy cure for that; I just cut a 3rd. O-ring seat, a bit shallower, between the factory two, which works with the same sized o-rings but fits tight enough to seal. (You'd need a lathe and a 4-jaw chuck...and realize the gas passage is close to the outside edge of the gas block.)
#4. Over tightening the two screws that hold the gas block in place can CAUSE a leak. It's a tube...if you squish it from the sides, it will grow bigger at the top and bottom. As an added headache, if you over tighten those screws, they tend to rasie burrs on the inside of the gas tube, which are sure to slice the o-rings when you assemble or take it apart. The only thing those screws do is to keep you from shooting the gas block off the gun, so just snug them firmly.
