But not in the front of the damn thing.
I was messing with a spare RS stock left from the RapidPistol build, when it hit me: There's room in there for a stick pack!
Those two Panasonic NCR18650B have nothing to do with this project, they are just a stand-in for the 2S pack of Sanyo UR18650SAX that will eventually be put here once I get my latest order of cells and pack assembly materials.
There is room available in these stocks, with some hacking and grinding, to fit larger cells. I don't have any loose Sub-C cells to try but I would guess they would fit. A-diameter cells like 18650 lithiums and 2/3A NiMH will fit without a doubt no matter where the wires are placed on the pack. There are also probably some expensive, airsoft-specific LiPo packs that could be employed in this setup.
Speaking of airsoft, that is where the idea of a stick battery came from since a lot of very similar needs exist in airsoft (desire to hide a big and capable battery pack inside a realistic and tactical-looking weapon's original design and interior spaces...), so if you want your RS to be a bit cleaner, better balanced and integration-ready, you may want to look into airsoft suppliers of battery packs for use in this mod.
A 3S of 18650s may be a little tight, but it might actually work out as well. I am going to use this on a stock-motor RS build so I am using 2S.