This is the most important comment. I have several bags, not just different shapes and sizes, but different uses.
Heavy and dense ones that are hard to mold but therefore also hold their shape, for benchrest stuff like sighting (I am not a BR shooter otherwise). Sand, and the heavier the better. Sandblast media is both crazy heavy and doesn't flow like sandbox or concrete sharp sand, if you want to go all the way, that is a good solution. Do be aware the finest sands will leak through many fabric bag materials so you'll end up with grit/dirt. You can try to sift to get rid of the dust but it will come back over time; you can line the bag with a plastic bag as well. Leather may leak at the seams but usually so little you can not worry about it.
I mostly lay in the grass or gravel and the rear needs to squeeze cleanly for height adjustment. Beans and poly fill work great for this. I only have beans in the old-sock bags so I remember they should never, ever, ever be wet. I have had my dried bean bags for 20+ years, no problems. I think airsoft pellets (and I do airsoft for FOF and training, so have plenty) are a bit TOO slippery, didn't like them as much as I expected.
A friend in the past had a walnut media bag, and it was right in between by my recollection. Which doesn't fit my needs (not stiff enough for benches, tiring to sqeeze) so I don't have one now. YMMV, based on bag material, bag shape, needs, how meaty your hands are, etc.
