Ok, mAh is the power of the battery. It takes about 1 mAh to fire one BB. Probably a little less. So lets say you have 1000 mAh, you will be able to fire 1000-2000 BBs. Voltage is what you need to look out for. If you use say, a 9.4 V battery in a gun thats only supposed to have 8.4V, you could do some damage.
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"Dang it Jack!!! Just because they're aliens, and their sculls are transparent, DOESN'T MEAN THAT THEY DON'T HAVE RIGHTS!!!!!!!! " -STARGATE SG-1