I use something like this hooked up to my co2 tank: http://m.grainger.com/mobile/product/LEGACY-Air-Gun-1KUD9Also, if you just pull that trigger when it's not attached to a keg, CO2 comes out so fast it freezes at the nozzle. You'll think the cartridge is empty - until you take it apart. Then the cartridge will rocket across the room as soon as it breaks free. Not that I've experienced this... umm ... a friend told me.
You can use the aforementioned keg charger to purge headspace but it comes out quite fast and those 16 gram cartridges can get expensive. If you tried to purge a bag of hops with it you'd have nothing in the bag but CO2!
Years ago I barely escaped injury when someone thought their cartridge was spent. When they went to replace it, the cartridge rocketed past my head and smashed an empty beer bottle on a shelf!