Cool video. I've never seen kinetic sand, it makes me want to go buy some to play with! lol Any idea if Bondo would be good to try? I have one of the big cans that's just been sitting in my garage for years.....a little more permanent than the sand. People use it as a wood filler so it's kind of the idea. Not sure if it would expand and crack the stock open? If you've got some input feel free to comment. The target down the road from me sells 3lbs of the sand for $11.
There's a guy on YT that has done it and the POI doesn't change through multiple brands of pellets/slugs. Damn impressive. The unit is light enough as is that the added weight is negligible.