Is your rifle a standard milspec barrel and fore end? If you have a custom built rifle, or one with certain options, you may need a barrel nut wrench specific to the fore end's (barrel guard/rail) manufacturer. For instance, Geissele rails require his barrel nut in order to mount to the rifle, and the wrench is different. However, when you buy one of his rails, you get the nut and wrench. I'm not sure who else's brands are similar. The milspec barrel nut fits many aftermarket rails, but check for compatibility.