Filling hollow tubes of my component racks

YES. Well , maybe. I am a dealer for Star Sound's Micro Bearings and this is a danger with them, they are exceedingly dense, much more than Atabits which I also handle. After I think about it you will probably not be able to get enough in to over damp it. I doubt if you will get enough gravel to do any good, I used cat litter for a while but abandoned it. I put 2 quarts of Micro Bearings in the target stands I use for my speakers [ one in each] and it works quite well, also have it is my Star Sound rack , I think it would work well in yours as it is very dense and heavy, think steel sand. But just start with a quart in each, they use to fill speaker stands full with it and I listened to one of them. it did "kill" the sound. Someone advised using steel blasting shot, I have never done this but it seems reasonable as the Atabits appear to be small steel shot and they work, but not as good as the Micro Bearings. I would give the gravel a pass.

Thanks Stan
I found a Mexican stone mix 1/16"-1/8" in size not really a pea gravel.

Stan what was the difference between the clay and the steel?

Custom made to your specs Carbon fiber rods. Use a a fuid silicone teflon lubricant and a mallet to bang it in.
For extra ballast use lead pucks encased in something no one can bight into. To save weight use memory foam torn into pieces, then tamp it down some and place a myrtle wood weight on that. Or potted heavy steel rebar like some waxes or epoxies. Even sound proofing green glue should work but will shrink and need a couple of applications. Just pour it over the metal bars . I guess that potting stuff they put trannies in is good .
BTW anyone know where to get the audiophile epoxy in question? Its exactly the same as the non audiophile self hardening goo,but costs 900% more per unit. You know the R and D burn has to be paid some how. Get the metal shop to make it your own braid and geometry. Use as precious a metal as you can like brass and bronze or damascus steel. This will help justify the obscene cost.
Silicone might feel better if a bit stiff.

Mechans
8 years ago I would have used some old gold coins and just filled the tubes but as they are the only thing worth anything today do you think some old GM stock certificates dipped in West Systems Epoxy and stuffed into the tubes would give adequate dampening.