I currently use the Vltor collapsible stock with the battery compartments. I have two AA lithiums loaded on one side for my EOTech and three 123 lithiums on the other side for my Surefire. The problem, minor as it may be, is that there is a small void that is not filled by the batteries. I am curious what other members are using to fill this void to stop the front to rear rattling. Thanks. Paul

I used some soft foam from a piece of foam that was removed from my pelican case foam when I cut it out.  It works perfectly.  I have seen other threads about this where guys use the tubular soft ear plugs to fill the voids.  That seems like it would work well also.

Ear plugs.  You might also want to mod your end caps by drilling a small hole through them and using knotted paracord to tether them to each other.  That way you don't end up with a loose cap.  Put some RTV on the inside around the knot to make it waterproof.  Also helps with the rattle.

Quoted: Ear plugs.  You might also want to mod your end caps by drilling a small hole through them and using knotted paracord to tether them to each other.  That way you don't end up with a loose cap.  Put some RTV on the inside around the knot to make it waterproof.  Also helps with the rattle.

I did this also and I am very glad that I did.  It works really well.  You can just pull one cap off and let it drop, not worrying about losing it.  It was very easy to do and a very worth while mod.

Can someone post a picture of the tethered end cap arrangement. I've heard it works well, but don't know if you're running the cord through the tube or on the outside. On the outside, it's gotta hang up on things. It it's installed on the inside of the tube, do batteries still fit? PIC NEEDED!

Quoted: Can someone post a picture of the tethered end cap arrangement. I've heard it works well, but don't know if you're running the cord through the tube or on the outside. On the outside, it's gotta hang up on things. It it's installed on the inside of the tube, do batteries still fit? PIC NEEDED!

Here is the link to the page where I got the idea:
LINK
It is under the "AR Furniture" section

Here is the Photo:
PHOTO

Hope this helps.  The cord in the picture is fairly short, you can make is as short or long as you like.  I made mine a little longer, though I can' really see what it would snag on.

Quoted: Can someone post a picture of the tethered end cap arrangement. I've heard it works well, but don't know if you're running the cord through the tube or on the outside. On the outside, it's gotta hang up on things. It it's installed on the inside of the tube, do batteries still fit? PIC NEEDED!

On the outside.  The cord is just long enough to hold the caps with very little slack between them.  The levers on the caps hold the tether down, and it can be tucked in the space to the rear of the caps.

Here's the tether untucked so you can see how short it is.

Here's one cap off, it doesn't hang real far.

Great pics! Thanks for the input.

If you wanted, you could route the tether through one of the holes on the underside of the stock where the screw would pass through.

Modified the mod a little bit.  If you use a slightly longer tether, you can attach it to the stock.
Place one knotted end of the 550 cord through the battery cap as before, but leave a lot more length than you will think you will need, and don't tie the other end yet.
Remove the battery compartments and you will find a small notch on the inside.  Route the untied length of tether through that notch to the rear, and pass it through one of the lightening holes in the spine of the stock.  Secure the battery compartment by holding it in place.  Then route the tether back forward through the notch on the other battery compartment and snug it back along the stock body.  Insert the screws and nuts and tighten slightly.  Play with the length of the cord so that the battery cap that is already secured has enough slack to be pulled out and inserted without difficulty.  You just want enough tether length to allow you to remove the cap and have it hang freely, you don't want 6" of cord hanging around.  Once you've got the tether length comfortable tighten the screws to where you want them.  
With the untied length hanging out forward, thread it into the battery cap as before.  Measure out a similar length to the other secured end, and remember to make it a little longer so that you will have enough left to knot inside the cap.  Cut the cord, burn, and knot.
Even with the tubes tightened, there is still some play in the cord and you can even it up if you're worried about it.  The extra slack in the tether can be tucked back into the voids of the stock underneath, and the levers on the caps will help hold it down.