I have noticed a few guys at the range that had what appears to be a bean bag mounted on the side of the butt stock on their rifles. What is the purpose of this piece of equipment and what are they called?
Sand sock, Bean bag, Rear bag, It replaces you using your hand. Its for holding your rifle while in the prone or on a bench.
Use the nonfiring hand to support the butt of the rifle. Place your hand next to your chest and rest the TIP of the butt of the rifle on top of your hand. Ball your hand into a fist to raise the butt of the rifle or relax your fist to lower the butt of the rifle. <span style="font-weight: bold">A preferred method is to use a sock filled with sand or a small sand bag placed in your non firing hand and squeeze it to raise the rifle butt and release the bag to lower the rifle butt. Using this sock or bag method lessens body contact with the rifle and can eliminate an added human variable.</span>
In short you use it to adjust your point of aim up and down by squeezing or releasing the bag. I use a G.I. issue sock with air soft pellets and it works pretty good. But that's the way I was shown. These high speed low drag things are very helpful and there are quite a few to choose from. Triad Tactical also has other choices and there is some others as well.
that is a little different than what I saw, they guys who had them also had rear bags that they were using. I was thinking it might have something to do with adding weight to the stock thus reducing recoil in some way.