GBB model: TM mk23 nbb (I'm the second owner)
propellent: I've put through at least 2 cans of propane, got the gun used and it was always on propane. probalby another couple cans there.
Failures to date: none at all (gun wise). no signs of wear or problems other then the common mk23 mag presure valve went after the 2nd can of gas (by me). used 5 minute epoxy on and around the main seals and valve. gun is still working 100%
Generally for CQB i've been useing this gun as my main gun. I go through a good 5 - 10 mags of ammo in a game night. good point about propane is because the gun is so efficient with it's gas I can use 3 mags of of ammo with one fill up with propane.