134a gas in pistol...
Generally here it goes:
Efficiency of the manufacutre:
Upper Slide Weight:
Plastic>Heavy Weight>High Grade Aluminium(prime)=Aluminium (KSC Taiwan) >Metal(WE as example)
So basically if you got a plastic slide pistol made by japanese company chances are it will get a good efficiency, 2 is probably a good number, maybe a lil bit more.
TM, KSC System 7 are really efficienct and it will be able to push 2 mags out. (assuming no rapid fire)