Eye Relief is the distance needed between your eye and the read most lens of the optic, called the Ocular Lens. This distance is set based on the shapes and spacing of the lenses inside the optic. This distance is the optimal place to have your eye in relation to the optic for the best field of view and lowest amount of parallax while shooting.
Our Tomahawk line has eye relief of around 3.8 inches, while our newer Arrowhead models have an eye relief of around 3.5 inches.
However, it is important to remember to set any magnified optic for your own eyes. You can achieve this by figuring out the approximate distance you want it mounted forward on your carbine or rifle, and then while it is still loose in the rings or mount, shoulder or bench the setup and slowly slide the optic back and forth in the rings.
When you have the best possible distance for your eyes, make a mark at the junction of the scope and ring shoulder. Now as you work on tightening your rings and leveling your optic, you can make sure your eye relief stay the same.