Mine are filled with split yellow peas. Cheapest ballast I could find that had some mass to it, not too small and the bags hold their shape well. But heavy, so if you need to carry them any distance they may not be ideal.

Hello there,
I have filled ours with cat litter ... the re-cycled newspaper type which you can buy online or in pet shops and it is called: :
Breeder Celect

There is also virtually the same product used for rabbit/ferrets/guinea pigs etc. only this is sold as:
Back-2-Nature

The pellets are paper but it's not a problem for me as I don't go shooting out in the rain and if you are the same or your gun rest bags are water proof , then this would not be a problem either.

I went to the pet shop and got vac packed saw dust ,cheap goes a long way and don't turn to dust o and it smells nice !

Aldi Rice

Another handy hint: if you use your bags outdoors or in the damp empty a can of Scotchgard into whatever you put in the bags and they become completely moisture resistant. Got this from my neighbour (he's a deer-stalking, fly-fishing merchant banker type) after he left his rest bag in his car over a long wet weekend and the rice expanded, split the bags, poured out all over the floor and attracted rats. Many rats. And it was a new Range Rover. It isn't new any more.

Filled mine with LDPE poly beads. Not too heavy or light, doesn’t absorb water and as such go mouldy like rice etc can. About £18 for 5kg.

When I was deployed overseas and now back State Side I like to use a D ring to clip my rifle to my body armor. With the right adjustments the rifle will not block any of the items on your armor and it sits just below the pocket of your arm where the butt stock would be placed for firing; not to mention it doesn't come into contact with your ruck at all.

https://accuratelyshooting.com/best-hunting-backpacks-with-gunholder/

Oil Dri Concentrate Floor Absorbent
$18.43 for a bag of 25 Lb

https://smile.amazon.com/Oil-Concentrate-Floor-Absorbent-Bagged/dp/B00GP57QVU/ref=sr_1_2