Shelledpants got it on the nose.
Your problem is basically a battery a bit too strong for the setup. A lot of rotational momentum is built up and the sector gear just continues to travel for a bit until the spring finally slows it down. The anti-reversal latch then keeps the piston and sector cocked back to however far it was able to turn.
Keep it down to an 8.4v mini and it won't over-rotate so much or use a smart logic chip with active braking controls to stop the rotation as soon as the shot is fired.