I have been using old shot bags filed with rice for years. At first I would open them up periodically because I was nervous about moisture, but never found any problems. However it is not a bad idea to use plastic bags just in case.
Sand doesn't absorb water any more than larger rocks absorb water. Sand granules are tiny rocks. If a leather or canvas shooting bag is filled with sand and the bag is soaked in water, it will remain wet for some time because the water is "trapped" temporarily by: (1) the bag and (2) the volume of sand. Sand works very well in small properly designed shooting bags made of leather. The weight of sand-filled small leather shooting bags makes the bags very stable. It is a good idea to use plastic bags inside shot bags because those canvas bags are much more porous than leather bags.
I have started using small aquirium gravel works great and no dust seeping out like sand . I have heard from some that say they use air soft bb's but I dont think they would make a solid rest . I may try them in a small bag to make a light weight tactical carry bag that will fit in a cargo pocket for prone shooting to put under the stock .
I tried sand , but it always found some small way to seep out. Changed to rice 5 years ago and haven't had a single problem. I don't let them get wet . The bug thing has been a non-issue for me.
RFOLMAO. I'd guess it would be about a kilo per bag and at three bags, that's 3 kilos or 6.6 lbs! Let me know when you go to the range. I won't touch your guns but I'm grabbing your sand bags.
BTW, I'm a peasant and I use sand in mine. Rice is for eating.