The mag is what contains the gas and is what leaks. Most lower end manufactureres have bad seals in their mags and low gas efficiency. If you had say a KWA mag in an HFC (which I don't think would fit, but hypothetically), you would probably have no leaks and be able to shoot off 2.5 mags in one fill of gas.
Again, don't buy a KWA mag for your HFC, I don't believe they are compatible. Just making a point.