A machining tool assembly is provided for machining a blank to form a firearm lower receiver. The assembly includes a pair of side plates positionable on opposed sides of the blank to capture the blank therebetween. A template plate having a template aperture is adapted to be removeably attachable to the side plates. The template aperture is aligned with a top surface of the blank and is adapted for machining a cavity within the blank having a peripheral contour complimentary to a peripheral contour of the template aperture. A centering mechanism is adapted to be removeably attachable to the template plate, with the centering mechanism including an opening aligned with a corresponding void formed on the blank to align the blank with the template plate.