For firing guns, I no longer recommend servos at all, I suggest the use of solenoids and electronic switches to operate them. The KIP solenoids are a good choice, and are available in 6V versions. The clippard maximatics also work but need a higher voltage (the test one I have stops working reliably at about 7.5 volts give or take) and are rated at 12V For rudder and any other servos, check out the waterproof servos from traxxas. I suggest ebay for buying them. http://www.traxxas.com/products/accessories/trx_accessories_servos.htm