A bayonet mounting fixture can adapt a rifle having a mounting rail to also have a bayonet mount. Previously, bayonets were often mounted to a firearm by passing the barrel through a hole in the guard and then attaching the rear end of the bayonet to a bayonet mount or mounting lug further back. The bayonet mounting fixture can have a forward mount and a rear mount attached to the mounting rail. The forward mount has a round piece that can go inside the hole in the bayonet's guard. The rear mount can have a mounting lug to which the rear end of the bayonet is attached.