Image Number 3 for United States Patent #6591533.
A locking hinge especially used in a folding shoulder rest for paintball guns. The locking hinge comprises a first hinge plate and a second hinge plate in relative rotation. The first hinge plate has a hinge joint axis, a plurality of symmetric mounting holes, and a counter bore for receiving the locking mechanism. The counter bore in the first hinge plate contains a spring and a locking cylinder. The locking cylinder has a taper on the end directed toward the second hinge plate. The second hinge plate has a relative hinge joint axis, a plurality of tapered circular cavities or notches, and a means for symmetric mounting. The locking mechanism sufficiently aligns with the tapered circular cavities or notches at plurality of predetermined positions thereby preventing rotation of the hinge plates. A minor misalignment between the locking mechanism and tapered circular cavities further provide rigid locking of the hinge plates.