When I was involved in making shooting bags we used recycled plastic pellets from a local industrial plastics company.
They were similar to the plastic pellets found in craft stores.
There is also a light plastic fill called G-Lite that is popular where weight is a consideration, but this fill isn't cheap.
If you are shooting from a bench, not having to carry your bags around, and want something really solid you can use lead shot.
I had a leather bunny ear bag filled with lead shot and it was SOLID!!!!! But it was for bench/Prone use only and not to be packed around.
You want a fill that is not going to be affected by moisture/water.
They were similar to the plastic pellets found in craft stores.
There is also a light plastic fill called G-Lite that is popular where weight is a consideration, but this fill isn't cheap.
If you are shooting from a bench, not having to carry your bags around, and want something really solid you can use lead shot.
I had a leather bunny ear bag filled with lead shot and it was SOLID!!!!! But it was for bench/Prone use only and not to be packed around.
You want a fill that is not going to be affected by moisture/water.