In a previous video (Ep 222), Jeff explained how to mount a red dot sight on an AR15. Now, he explains sighting it in or "zeroing" it, using his Aimpoint T2 as an example. First, decide what you want your ZERO RANGE to be (see Ep 225): 25, 50, 100 yards, or farther? You'll also need to know the CORRECTIVE VALUE of your sight's adjustment knob. Jeff's T2 adjusts ½ MOA (minute of angle) at 100 yards per click of the knob, so to move the point of impact 1" at 100 yards, he'll turn the knob two clicks. If you've already sighted in your AR15's iron sights, you can just cowitness the red dot so the dot lines up on top of the front sight post. Otherwise, start by adjusting windage at 10 yards. Once you have the sight dialed in at that range, move out to your zero range and finetune it. The first correction should be a "major" adjustment: if the shot is landing 2" to the left of your point of aim, dial the sight to move it 2.5" to the right. If you now overshoot, it's easy to dial back a smidge. As a bonus, you'll learn the left and right limits of your aim point. When you're done, confirm your zero by shooting one more group. Reconfirm every time you start a range session, switch to different ammo, or make some change to the rifle.