Having a rear bag was one of the biggest factors that helped me get ‘in tune’ with long distance shooting rifles. Since first using one it’s now something I use with all my target rifles and an essential piece of kit in the range bag.
The bag itself sits under the rear stock and helps in manipulating the elevation. The idea is to reduce the amount of muscling needed to get the reticle onto the target. The bags are usually filled with plastic beads and made from a durable nylon.
You want the right size bag so you don’t have to squeeze the bag super hard to get on target. The less pressure on the bag the better. You can use a rectangular bag in three different height variations including the flat, long side and short side.
Once your reticle is settled on the target using the bag, try not to fire right away but take your time to breath in and out for a couple of cycles and fire at the end of your breath on the natural pause.
I have a small and large Tab Gear rear bag that I have been using the last three years. Having a small and large bag means I can always have the right bag size on hand.