A hinge joint includes first and second hinge leaves pivotably mounted on a hinge pin, where the hinge joint is operable between open and closed positions. The hinge joint has a locking mechanism that prevents opening the hinge joint from a closed position until the locking mechanism is moved to the unlocked position. In one example, the locking mechanism includes a plunger retained in the hinge pin. When the plunger is depressed, one or more protrusions can move from a blocking position to a non-blocking position. In some embodiments, the locking mechanism must be moved to the unlocked position prior to opening the hinge joint. The locking mechanism may be used alone or in combination with engagement structures between the first and second hinge leaves. The hinge joint can be part of a folding rifle stock assembly or other hinged assembly.