Frim what im aware they take a large type battery, and unless you have downgraded the battery you should be running an 8.4, what are you trying to get out of your battery?
Are you just wanting it to last longer.... or trying to boost your ROF?
Scroll down to the 8.4v packs, you won't want the hump back packs though. The higher mah the longer the battery will last, so it is just a question of what you can afford.