About twenty years ago, I got hold of a yard or so of heavy artificial upholstery suede. Made it up into bags, about 6x8 inches, and filled them about 7/8ths full with uncooked NAVY beans. I was tired of canvas sandbags sifting the finer particles of grit into my guns with every shot (as I suspect kitty litter, which is just clay chunks, may also do eventually.)
The 'suede' bean bags have proven to be durable and practical. I was a bit afraid that the beans might attract moisture and spoil somehow, but that hasn't happened. The consistency of the bags is firm, but adjustable, so you can really settle a firearm into them. I use them to shoot handguns and rifles, and no machine rest would be better.
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If they take our guns, I intend to let my hair grow long and acquire the jawbone of an ass.