Mounting Instructions: Backup Iron Sight (BUIS):The BUIS must be mounted in the rearmost slot on the upper receiver for the range adjustments and the zeroing procedures to be correct. Using the Locking Bar, PN 12996823, NSN 5305-01-484-7999, and the Recoil Screw, PN 12996824, NSN 5305-01-484-7075, align the Recoil Screw in the rearmost slot with the range scale facing rearward. Make sure the BUIS is flat on the receiver rail with the angled edge under the upper receiver rail. Tighten the screw until the BUIS doesn't slide forward or rearward by hand force, then tighten an additional 1/4 turn. Do not over tighten. The Recoil Screw has a locking patch on the threads so it must be replaced if removed.
To zero the M4/M4A1 Carbine, turn the range knob until the line on the sight cam on the left side is aligned with the 300 meter mark. Use the 25-meter zero target for the M4 Carbine and center your rounds by adjusting the weapon front sight post and the BUIS windage adjustment knob.
To zero M16A4 Rifle, turn the knob until the line on the sight cam on the left side is aligned with the white line between the 300 and 400 marks. This mark is a half notch location, so it's more difficult to align. Use the 25-meter zero target for the M16A2/M16A4 Rifle and center your rounds by adjusting the weapon front sight post and the BUIS windage adjustment knob.
**25 Meter Zeroing Targets can be accessed by downloading a copy of FM 3-22.9 Rifle Marksmanship M16-/M4 - Series Weapons from the Official Department of the Army Publications and Forms website located at: http://armypubs.army.mil/. The M16 and M4 zeroing targets are designed for the unique sight adjustments required for each weapon system.