Dot Sight

How To Zero A Red Dot Sight

Sighting in a holographic or red dot sight has much in common with zeroing a scope. Anyone can zero a scope by trial and error, but without doing so methodically you may waste a lot of ammunition. This method assumes you don’t have limitless ammo or own a boresighter and don’t have access to a formal centerfire rifle range.

The first thing to do is make sure 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), drawing back the bolt and manually checking the chamber and feed ramp. You must ensure that your red dot sight is level relative to the rifle it is mounted on. Many sights, for example the Eotech 553.A65 (United States Special Operations Command’s SU-231/PEQ (Model 553)), come with mounts that do this automatically. The tubular ones often have the feature that they can be rotated in the rings so that either adjuster can change elevation or windage/azimuth. The sights with tube construction can typically be rotated so that either adjuster can move elevation or windage to suit left or right handed shooters. 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. When locating the mounts you need to consider anything else you want to mount on top of the rifle, such as a magnifier or in-line night vision. If you then wedge 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. Eye relief (important with magnifying reflex sights) can also be adjusted at this stage by pushing the scope through the rings if clearance allows. You can then tilt the sight until it is straight and tighten the rings down, tightening diagonally opposed screws one turn at a time to ensure even pressure. Dial the elevation knob/dial right down, stopping as soon as resistance is met, then wind it all the way up, counting the clicks as you do so. Now wind the adjustment back by half that number and perform the same procedure with the windage adjuster. The sights adjusters are now centered. If the sight is now pointing way off target, it is either broken or, more likely there is something wrong with the way it is mounted.

You now need to find somewhere to shoot with a safe backstop and at least 100, and preferably 200 yards/meters distance. It’s helpful if it incorporates a much shorter range with 25 yards/meters being perfect.

A calibrated target will help, especially if you have a spotting scope.  All you need to do is draw a grid of known dimensions on a piece of card.. A one inch grid will work well for sights calibrated in inches or centimetres. The lines need to be drawn thick enough to be seen at 100 yards through your spotting scope. The aiming marks should not be obscured by the dot of your sight. For the 2 MoA dot of an Aimpoint, your aiming marks should be 4 inch diameter at 100 yards and 1 inch diameter at 25 yards. Whilst you should get by with one such target with five to ten aiming marks, it is best to have a few spare. Try to get to the range on a calm day and try to avoid the hotter hours to prevent mirage becoming a problem. Take some binoculars, pen and paper and a pocket calculator to the range with you!

You start by getting the rifle to put its shots on the card It doesn’t take specialised equipment to do this. If you have 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 red dot on 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 the target, 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 enable you to calculate the adjustment to zero your red dot sight.

It could be that you have a red dot sight calibrated in an odd way. The Trijicon TA31RCO-M4CP (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 CompM4s (the new US Army M68 CCO) has clicks that move the mean point of impact by 16mm at 100 meters. It also isn’t always possible to sight in at convenient 100 yard/meter increments thanks to terrain constraints. Lets say you need to zero your M68 on a range that is 30 meters long. Take the distance in mm one click moves your point of impact at 100 meters and divide that by 100, then multiply the result by the actual range. 16 divided by 100 is .16 and .16 multiplied by 30 is 4.8 which is your click’s value in mm at 30 meters. Let’s say your groups are 87mm off to the left. You divide 87 by 4.8 which gives you 18.125. Disregard the .125 and round down to 18. You now turn the windage dial 18 clicks to the right and you are on target in azimuth. Follow the same procedure with your elevation and you’ll be on target.  Fire another three round group to check the result. This sounds like a lot of work, but it means you use the minimum ammunition to get on target and the technique will work at any distance.

You may have iron sights to co-witness. Don’t fall into the trap of thinking the red dot has to sit straight above your foresight – as long as both the red dot and the iron sights are zeroed it doesn’t matter if they are apparently out of alignment with each other. You now need to check the result at a longer range. If your red dot sight is low mounted (i.e. not on top of an AR15 carrying handle) and is chambered for a flat shooting round like the .223 or .308, a 25 yard zero will have put you on target at 200 yards, 2 inches high at 100 and 6-8 inches low at 300. If you only have access to a 100 yard/meter range your next task is to get the rifle grouping 2 inches high at that distance. Do so by firing three three shot groups and again taking their mean variation and using the calculation. At 200 yards each click will move the point of impact twice as far as it does at 100. Manufacturers will normally publish the ballistics of their loads giving their trajectory and wind deflection values – these figures are perfectly good enough 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 good idea.

Copyright Chris Pieterman 2010

Leatherwood M-30 Red Dot Sight