Just bought a set of BPS Front & Rear Shooting Bags to use at the range for sighting in my ML and rifles.

The directions say to fill with
-dry silica sand
-kitty litter
-rice or
-ground corn cob tumbling media.

I dont have any of the above so will be headed out to buy one of these options. I am guessing that sand would be the heaviest but also the cheapest?

What have you used, and which works the best?