US Patent 9568261 Firearms receiver with integrated locking joint

Methods and structures for tightening the fit between upper and lower receiver subassemblies in a firearm comprising the addition of a raised embossment (projecting upward from the surface) on the upper side of a lower receiver walls or the underside of an upper receiver walls near the pivot pin. When the upper and lower receivers are connected by engaging the rear take-down pin the embossment leverages the receivers on the pivot pin to provide a tight fit.