Just one note (my own opinion) to add to Ricochet's post:
I find the trigger response on the stock VFCs to be a little lacking (and the rof a bit too) when using 7.4v lipos. If you get one of the new VFC line, they'll have mosfets already installed and you can go straight to a 11.1v lipo battery which will help in the trigger response and rof. Otherwise, it's a bit more work to wire in a mosfet, but doable and worth the effort in my opinion to be able to use 11.1v lipos without fear of shorting out your contacts.
Custom VFC M4 - KJW KC02 - KJW Mk1 - TM P226 - KJW M9