The Daniel Defense Big Hole Flat Top Upper is designed and built with a .315 front pin hole to fit the early Colt lowers with .315 front holes. This eliminates the need to use an offset pin, which requires 2 screwdrivers during assembly / disassembly of these rifles.  This upper comes with our .315 Push Pin, which makes the assembly / disassembly of the upper and lower quick and easy. It has a detent ball to hold the pin securely in place.