Well a fully upgraded airsoft gun, at 400fps, firing around 1200rpm (really fast), with a tuned systema magnum (draws the same if not more than an untuned motor), will draw ~17A. So it's safe to make all your guesstimations to 25A for high power setups, and 20A for normal 400fps, 1000rpm setups.
If your LiPo doesn't supply enough amperage for you motor, your LiPo goes up in smoke.
Although every 7.4v 20C LiPo has the same output, there IS a factor than can cause a change in ROF on a gun; and that's the wire gauge of the battery.
If you have two batteries, one with 18 gauge leads and one with 12 gauge leads, the 12 gauge is going to offer a higher ROF because the wire has less resistance in it.
For example, I have 3 different 7.4v 20C 1600mah LiPo batts, a turnigy and 2 intellipacks. All having differing rates of fire due to the 20, 16 and 14 gauge leads.