I dont know how other people have done it, but this is how i did it. First you buy this: http://shop.ehobbyasia.com/king-arms-li ... -pipe.html you cut off all but 1 inch of the reciever's stock tube to fit it on. The wires and the battery will all fit into this new buffer tube and voila, you're done.
I have that King Arms Buffer Tube on my VFC 416 and also run the LiPo (i think) the OP is talking about on a standard stock buffer tube on my HurricanE 416. the only issue with running that battery on the OP's post is that it will be too long to close the endcap. Even with the King arms buffer tube that battery will be too long. While that battery will in all actuality work, in order the make sure to fit the battery in the buffer tube you need to use the King Arms Lipo designed for the tube with the Lipo tube to really make it work. No matter which Buffer tube you use, you'll need the wires to be longer then the tube to be able to connect the battery in such a tight squeeze (yes even Deans is a tight fit). (see following pics). Also take note that using a battery that sticks out past the end of the buffer tube (also take in account the wiring) will cause you to not be able to close the stock all the way and exposing the battery and wiring depending on your stock. (I used duct tape on the end once and also a SOG stock pad to cover it). The King Arms battery is shorter and even still in a standard buffer tube this too will stick out which is something to think about (explained in pics below as well).
Size difference in batteries:
This illustrates how far that the Firefox battery will stick out with standard buffer tube and how far stock can retract with out exposing wires.
This picture show the shorter King Arms battery in the standard Buffer tube, note that the end cap will not close after the wiring will be connected yet it can close more but still not fully.
Using the King Arms Lipo buffer tube and king arms battery allow you to close the endcap on the buffer tube and retract stock fully without any exposed wiring.
And side by side compare or the difference in retracted lengths, Top one is standard Buffer tube and the bottom is the King Arms Lipo tube. take note that the top gun with the standard tube has the best case scenerio with the King Arms battery (I know that one is not clicked all the way in, found out after pic taken and too lazy to retake)
I hope this helps