This used to support the rifle on the bench. They are adjustable usually for height and windage (left-right). These can be of any weight that you want, heavier the better, but you do have to carry it around
Front & Rear Bags
These support the rifle . The front bag is attached to the Stand (see above) and the rear bag sits on the bench. Both bags are normally sprinkled with “Baby Powder”. This allows the rifle to have constant recoil so that the bullet is not being deflected as it leaves the barrel. Both bags can only be filled with SAND or a like material. Heavy Sand which is only Grit Blasting medium is the way to go as the grains have sharp edgers that lock together and gives consistent pressure on the rifle.