dude... let me explain to you...
Higher voltage will make your motor cycle fater
Higher mah (miliampérage) will give your motor a higher energy = higher torque (but could also be influenced by the battery chemistry)
With a 8.4v 1400 MAH battery, (and I guess a near 400 fps spring?)...
your motor doesnt receive enough energy from the battery to remove the gearbox gears and spring from its inertia = problem in SEMI
A lipo battery, however, because of its chemistry, gives a lot of energy/torque to your motor, and it creates a really important electric ark at the switch assembly, each time you pull the trigger, which creates a black carbon residue. A MOSFET will act as a electrical switch instead of a mechanical one = no carbon residue!
Unless you want to open your gearbox everytime after you've shot with your gun to clean the carbon residue, or experience the frustration of a non shooting AEG in a middle of a game, INSTALL A MOSFET WITH YOUR LIPO!
"A workout isn't easy, but neither is being fat, make the right choice"