As the name implies, sand is the best filler. Why? Because sand is moldable to adjust the stock of a rifle when shooting groups. When using a front bipod or tripod rest, the rear bag is what controls elevation for that last tiny adjustment. How? By the non trigger hand squeezing that sandbag to raise the crosshairs above the bullseye, then relaxing the grip to lower it right on.
Sand is also heavy. DUH! That means it is also more stable, and stability is what you want when shooting off a bench
It is of utmost importance that it does NOT leak from the bags. Sand is as hard as glass, it's the worst thing you could get in the action of a firearm. When I'm making sandbags, I first put it is ziplock bags, then tape them shut. Then a strong thread is used to sew them shut.
My buddy's wife works at a bank. She has access to old coinage bags. They're much bigger than shot bags I have, they make a wonderful rear bag for rifle shooting.