Before filling the bags, you can put a plastic bag inside each. This keeps the filler from abrading the seams. Just tape the end of the bag when finished filling and stuff the bag into the fill hole.
What to fill: You can fill the bag with rice, sand or cat litter, the best choice is polishing media, it doesn't smell bad like cat litter, not attracting pest like rice and not heavy like sand.
Don't overstuff your bags, leave a little space so the rifle can get cradled in front. This helps with repeatable accuracy. Your rifle only moves straight back with no horizontal movement and minimal muzzle jump.
How to fill: You can make a funnel from poster board or ever paper and tape it together then use a plastic flexible cup to fill.
This set is light enough and small enough to toss in the car when you visit the range, and it will do the job well. They can be easily refilled (or emptied) so you can adjust it to your preference.
