If your battery is orientated this way it might fit, the sides hold 3 sub-Cs each and the cap has a slot which looks like it can accompany 2 more.
I didn't pay much attention to these groves and always put that single battery in the buffer tube, so this didn't allow me to get the stock in its 1st or 2nd position, which was a turnoff. Sadly I figured it out AFTER I sold my 416. BAH!