Know your height

“Lower third, absolute height” doesn’t mean anything to you? These are different mounting heights for optics. The designations indicate where the iron sights are visible in the Red Dot. So “Lower 1/3” means that the front iron sight is visible in the lower third of the optics. With “Absolute Co-witness,” the red dot and the irons are visible at the same height. However, there are also other, higher mounts available.

These always refer to the distance from the optical centerline to the top edge of the rail, and that with an AR-15 with a “Flat Top Receiver.” The most common heights are:

• Absolute Co-witness: 1.4" about 36 mm (not in the image)
• Lower 1/3 Co-witness: 1.6" about 40 mm
• UNITY FAST Mount: 2.26" about 57 mm
• HYDRA Mount: 2.91" about 74 mm

Of course, there are also intermediate heights, such as the SCALARWORKS SW0120, which is 1.93" about 49 mm high. 

The HYDRA is the tallest mount and is intended for those who mainly shoot with a gas mask and night vision goggles. The FAST mount is for those who occasionally want to shoot with night vision goggles and prefer a more upright head position. Both have the advantage that the standing position works great (head vertical), but the disadvantage is that any prone position can become uncomfortable for the neck.