A locking block mechanism for a compact semiautomatic pistol includes a pair of forward guide rails, which are normally separate from the locking block. Additionally, the locking block includes transverse ribs on the sides of the locking block, which engage mating slots in the polymeric frame of the pistol and secure the block to the frame. The guide rails and transverse ribs are separated by a space or discontinuity on the side wall of the locking block. The guide rails include chamfered front and rear edge surfaces and arcuate, convex bottom surfaces. The above-mentioned features function to create a durable locking block that allows for the construction of a pistol that is more compact than prior art designs.