well the higher the voltage on the battery the more current gets to the motor which makes the motor turn faster which makes the gun shoot more rounds a second. a 9.6v battery would have a faster rof than an 8.4v battery. But internals also effect the rof of your gun. One of the main things is the motor that is in your gun, if you go with an upgraded motor but stick with the same battery you wouldnt get a faster rof than if you were to go with an upgraded motor and with an upgraded battery. But personally I just went with a 9.6v battery which did plenty for my rof. and if your gun still turns over but doesnt shoot out bb's you could have a feed problem. try a different magazine if you can. And maybe try to fire it on semi, then once it stops firing, turn the gun upside down and try to fire it again and see if a bb comes out. If it does then it is mostly likely a feeding issue