[Naglfar]

The motor in that pistol will not be current limited by the shittyness of the stock battery if you put in a good one. You won't necessarily see it wear out much faster but you will notice a massive ROF increase, even with the same voltage due to the larger amount of current a good battery can put out.

Scientific explanation:
Current is charge per second, charge exerts a coulomb force on the commutator of the motor. Increasing the amount of charge on the brushes (i.e. higher current) will cause the motor to spin faster. An increase in voltage will, on the other hand, increase the force of the charges, meaning you get more torque. This is why a higher voltage can pull a bigger spring, and why putting more current through the motor will make it turn faster.
A good battery has less internal resistance than a shitty one, simply put. Though I do not know much about the chemical properties inside the battery that contribute to it storing more charge or having a higher current rating. I do know about what is happening inside your circuit.