So, after seeing the eBay ad for rubber beads, on Saturday I ordered the 22lb bag for $50, which included USPS Priority flat rate shipping.
Order arrived yesterday. I weighed the bag with sand before emptying it.
BEFORE: 37.9 lbs
The sand was dry as it poured out, so no moisture adding to the weight. Filled and weighed the bag with the new beads:
AFTER: 21.7 lbs.
16.2 lbs lighter, which is almost 43% lighter.
That means I can carry much more ammo now!!

It's a one-time cost, the rubber is not a problem with moisture, and if your bag is smaller, you don't need the 22 lb size. I had maybe 1/4 to 1/2 lb left over. So if there is any settling or compression, I can add more later.
