Just the barrel (on the old-type M4's) would wobble, but because the rail is held in place by the rear d-ring and the front retaining ring by the front sight (which is attached to the barrel) you may have some movement of the RIS unit, too. You *shouldn't* have a battery-storage issue with the "new-type" barrel as it is bevelled and stepped to accept a battery to begin with.
The ultimate cure (and will make your RIS solid as a rock, BTW) is to add a Laylax Nitro Rail, which attaches to the top of the RIS and goes straight back and bridges to the flat-top of your receiver. It make the whole thing "knock-down-trees" solid.
You can see a pic of the Nitro Rail/RIS combo here:
http://img.photobucket.com/albums/v5...ors/ecco16.jpg
I bought the actual rail and RIS from the guy in this picture, BTW.