AR series (M4/M16) weapons generally have outer barrels that are held on to the receiver with a screw-ring called a barrel nut. Although it can be tightened by other means, the barrel nut wrench is a specifically designed tool that makes it much easier to tighten and loosen the nut.
The barrel nut wrench works on the standard type of barrel nut but some specific kinds of rail systems require a special barrel nut as the standard one will not allow the front system to fit. In the case of such special nuts, you need a special wrench to operate it.
This wrench is for the Madbull Daniel Defense series, RIS II series (M4A1 and MK18) and Omega X series front set barrel buts. Madbull make another barrel nut wrench for other front set barrel nuts as well (MB-PWS-WRENCH).
This wrench is a single piece of steel with a dull back coat finish, just like a neat little tool grade item should be.