Replacement barrel anchor.
The 12 gauge version is shown in the picture. The Barrel Anchor comes in three sizes (12, 20 and 28 gauge). To install, you drop the Barrel Anchor of the correct size is into the muzzle of your gun, insert a common (flat tip) screwdriver in the end of the shaft, and then tighten the 7/16th nut with a wrench. Don't make the mistake of turning the shaft with the screwdriver. You must tighten the nut while holding the shaft stationary.