It's an adjustable gas plug, that replaces the USGI plug. It allows you to adjust the gas pressure to allow you to shoot various ammunition loads without harming the rifles op-rod. Alot of commercial ammunition is too hot for the M1 Garand, and it can cause op-rods to bend or break. If you shoot alot of commercial ammo, or you want to shoot a hotter load than M2 ball, it's a good investment. Most of the time you can find a commercial load, that won't harm the rifle. I have an adjustable gas plug, but I never use it. Since I just plink, I shoot M2 ball ammo from the CMP.