is the gun upgraded? and are you comparing two batteries that are the same when you say you hooked another one up...
No, the gun is not upgraded, totally stock. The other battery I tested it with was an 8.4v 1500 mah from a different mp5. Correct me if I'm wrong but from what I know about batteries its the voltage that has to do with the guns power and mah that is responsible for how long the battery will last. Thus both batteries being 8.4v should result in the same output of power given that the stock battery is holding a charge correctly.
it also depends on the output of the battery...
So you're saying there is no larger internal battery solution..??
battery box as in "AN/PEQ" type battery boxes. The TM M4 RIS has those for 8.4v 600mah mini batteries. CA, and ICS makes battery boxes. Battery boxes are pretty much boxes that look like a laser sighting system. Unlike battery bags, battery boxes are held on to the RIS/RAS by a little clamp thing that you tighten using allen wrenches or screw drivers or just hand tighten them now this is kind of pretty long but I feel like I should go on and if I do I'd be wasting my time and all that and if you're still reading this then I guess you're wasting your time too so do I stop now or do I continue? LoL