How to Sight in a Red Dot Sight
Sunday, January 30th, 2011
CHEAP DEER FOOD PLOTS-Hunting
Zeroing a red dot or holographic sight is similar to zeroing a riflescope. Zeroing in a scope without following a method is easy – if you have time and ammo to waste. This method assumes you don’t have limitless ammo or own a boresighter and can’t get to a formal centerfire rifle range.
The first thing to do is ensure that your hunting arm is made safe. Rifles are typically made safe by pointing the muzzle in a safe direction, detaching the magazine (if present), pulling back the bolt and manually inspecting the chamber and feed ramp. The sight must be mounted level on the rifle or the rifle will be canted, ensuring your rifle will only be on target at one range and will be off laterally at others. The Eotech 553 (United States Special Operations Command’s SU-231/PEQ (Model 553)) incorporates mounts that do this automatically. Tube format sights can be rotated in their rings so either adjuster can alter elevation or windage, making the unit ambidextrous. It can be difficult to get such sights vertical, but the adjusters can serve as a visual reference. First attach the mount(s) with the lower part of the ring(s) onto the rifle. In deciding where to place the mounts you also need to consider other optics you may wish to add, perhaps including a magnifier or night vision. If you then secure the rifle between sandbags or secure it in a padded vise and level it, you can drop the scope into the open mounting rings and screw down the top rings so they exert a very light pressure of the sight. Now you can fine adjust eye relief (important with magnifying sights) by moving the sight back and forth in the rings if the design of the sight allows it. You can then tilt the sight until it looks straight and tighten the rings down, tightening diagonally as you would with a cylinder head, to ensure even distribution of pressure. Turn the elevation adjuster all the way down, being careful not to turn once resistance is met, then rotate it all the way up, counting all the clicks as you go. Now wind the adjustment back by half that number and perform the same procedure with the windage adjuster. The elevation and windage adjusters are now centered. In the unlikely event that the red dot is way off target, the chances are there is something wrong with the way the sight is mounted on the rifle.
You now need to find somewhere to shoot with a safe backstop and at least 100, and preferably 200 yards/meters distance. It will help if you can also shoot at a closer distance – 25 yards/meters is ideal.
A calibrated target will be of great assistance, particularly if you have a spotting scope. Just draw a grid of known dimensions on a sheet of paper or board. A one inch grid works very well with all the calibrations used on red dot sights. The lines should be visible through your spotting scope at 100 yards. The aiming marks need to be big enough not to be obscured by the aiming dot of your sight. For Aimpoints using 2 MoA dots, your aiming mark should be 4 inch diameter for 100 yard shooting and 1 inch at 25 yards. Draw a few of these targets, each with at least five aiming marks on each as you’ll probably want some target practice after zeroing. It’s best if you go to the range on a still day and avoid the hottest part of the day to stop mirage causing problems. Take some binoculars, pen and paper and a pocket calculator to the range with you!
You start by ensuring your shots find their way onto the card. There are a few ways of doing this. If you own a bolt action rifle, you can take out the bolt, put the rifle in a padded vise or between sandbags and look down the bore to align the rifle at a distinct point 25 yards/meters distant. You can now make the sight adjustments to put the sight’s aiming pattern or dot close to the center of the target. With an AR15 you can do this by removing the bolt carrier from the upper and clamping the barrel in a padded vise. Though this will work with some other semiautos, it will not work with Garands, M1As etc. nor with most pump and lever actions.
A more expensive alternative is to fire and adjust after single shots taken from a rested/supported shooting position. Avoid resting the rifle on a hard surface or using a bipod – sandbags are best. Your shots should be on the card or close to it. Once you are on the card shoot a group of 3 shots. Measure the difference in height of each of the three shots from the center of your aiming mark, add them together and divide by 3, then repeat with the difference in windage. To be more certain, shoot a few more groups and average their results. This will give you the adjustment you need to put your shots into the center of the target.
It could be that you have a red dot sight calibrated in a way that is unfamiliar to American hunters. The Trijicon TA31RCO (the USMC’s AN/PVQ-31B Rifle Combat Optic for the M4 carbine) for example has clicks that are one third of an inch at 100 yards and the Aimpoint Comp M4S (the new US Army M68 CCO) has clicks that shift the mean point of impact by 16mm at 100 meters. To compound the problem, you could only have access to a range that makes use of natural terrain features and is thus some odd distance. Assume you can only zero your M68 at 30 meters. Take the mm that one click will move your point of impact by at 100 meters and divide that by 100, then multiply the result by the range you are sighting in at, i.e. 30 meters. So you take 16 and divide it by 100 which gets you .16, then multiply that by your range of 30 meters to give 4.8, which is how far each click will move your mean point of impact in mm at 30 meters. For example, your groups are 87mm off to the left. You divide 87 by 4.8 which gives you 18.125. You can round down to 18. That is the number of clicks you should turn to the right to get on target in the horizontal plane. Follow the same step by step method with your elevation and you’ll be zeroed. Shoot another three round group to check your zero. This may sound tiresome but it will soon become second nature and it will save you a lot of ammunition and frustration.
You may also want to co-witness your iron sights at this stage. Don’t fall into the trap of thinking the red dot has to float straight above your foresight – as long as both the red dot and the iron sights are zeroed it makes no difference if they are apparently out of alignment with each other. You now need to check the result at a longer range. With a low mounted red dot sight and a rifle chambered for a typical high powered rifle cartridge like the .223 or .308, a 25 yard zero will have put you somewhere near the target at 200 yards, two inches high at 100 and 6 to 8 inches low at 300. If you only get to a 100 yard/meter range your next step is to get the rifle hitting 2 inches high at that distance. Repeat the process outlined above. At 200 yards each click will move the point of impact twice as far as it does at 100. Manufacturers conveniently publish the ballistics of their loads giving their trajectory and wind deflection values – these figures are more than adequate for use with a red dot sight at the ranges these are normally used.
Even the best rifle and sight combinations can lose zero, especially if abused. It is good practice to check your zero before any hunting trip or competition and should the rifle or sight take any impacts. Checking zero after long term storage is also a wise precaution.
Kenneth Ardleigh is a gun enthusiast of long standing with a keen interest in military red dot sights. Visit his website to learn more about combat optics, including the Eotech 553.