Re: I need to add weight to hollow butt-stock
Bunch of ways to skin this cat. One method is to take a ziploc bag, weigh X number of ounces of shot or other weight bearing media into it, install it and see how it feels. Keep playing with it until you get the weight right. While 'installing' it to check, be sure and stuff something in it like a rag or whatever to keep the bag of shot stable and not flip flopping around inside the stock.
Once you get the weight figured out and are happy with it, get some spray foam insulation in the can at your hardware store and use the nozzle to put some in the stock while it's laying on it's side. then put your shot in while it's still laying flat so it is relatively evenly distributed and not all in one area and continue to fill in the foam until the stock is full. Trim excess and reinstall pad.
I have seen people spray the stock with mold release on the inside of the stock thus molding the foam/shot into a sort of plug that be removed for maintenance when necessary.
This adds weight you can remov, and gets rid of the hollow 'thump' associated with hollow synthetic stocks.
There are other methods but for the garage gunsmith this works well and is cheap, easy, and relatively fast.