Mosfet + re wire.
read my post, I use a 2200mah brick hidden in a P mag. Not for everyone though. Scroll about halfway down for the battery stuff.
As for the switch rating, it will have the maximum amperage and voltage ratings on the switch. You should be less concerned with max voltage and more with max amperage.
http://www.airsoftcanada.com/showpos...45&postcount=3