It seems crude to use diodes to drop voltage. Generally, you should use PWM to control motors.
You can use an N-channel MOSFET or an NPN transistor on the low side, connected to a PWM pin on an Arduino or other microcontroller, to vary the power more efficiently.
Those are all Google-able, if your circuits knowledge is a bit rusty.
EDIT: The problem with dropping voltage with diodes is heat. The extra voltage is dissipated as heat, and the more current the motors draw, the more heat is wasted. Laziness and ignorance are not viable reasons to waste 30% of your battery power.
I was intending to drop the voltage of the pusher motor only, and preferably in the simplest way. I'll look into other options, I was just under the assumption that diodes was common practice in a nerf rate of fire application.