Batteries get hot, even with top of the line stuff. And in my own experience, even a battery that comes with a new gun can be rubbish from the factory. Happened to one of my NiMH batteries, I think one of the cells is dead and the whole battery is off balance.
But as stated above, if you feel that it is getting TOO hot, you could also be having a problem with resistance / wiring. Check connections / places with solder for any thin interfaces or loose connections and or trigger contacts. And if you've got the know how, consider switching to deans' connectors, because tamiya connectors have a lot of resistance, which kills batteries and motors that have high current, quick on/off motors, like what airsoft guns have.
Also try checking out your gear box's shim job.