Lapping a receiver ensures a perfectly square and true surface for the barrel to mate against, thus enhancing accuracy. Upper receiver/barrel alignment is critical to accuracy. Also, honing the surface can produce optimal positioning of the barrel nut so ideal torque can be achieved along with proper gas tube hole alignment. Use with a battery powered or electric drill to square the receiver face on an AR-15. Helps align the gas tube through the barrel nut holes, helps align front and rear A2 style sights.