If you fill it full of very small granules you can get away with just about anything. It is why the ranges I have seen use plastic. I have seen some bags filled with larger lead shot, maybe even buck or lead sinkers?, and it was unstable. I have also seen bags with small lead shot but they didn't fill the bag enough and it too is unstable.
I often use a lumpy backpack, ammo can, or hood of a car as a rest and can achieve respectable groups and I am only an average shooter.
Sand, dirt, bb's, plastic beads, silica beads, etc. can all work if the bag can be filled tight. I bet even expanding foam could be used successfully if you could get it completely fill the cavity and keep a useful shape.