Zeroing Red Dot Sights
Zeroing in a holographic or red dot sight has much in common with sighting in a scope. Anyone can zero a scope by trial and error, but without knowing what you are doing you may waste box after box of ammunition. This method assumes you don’t have limitless ammunition or own a boresighter and don’t have access to a formal centerfire rifle range.
It is vital to first ensure your weapon is safe. With a rifle, keep the muzzle in a safe direction, remove the magazine (if detachable), work the action and manually inspect 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 (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 models with tube chassis can typically be rotated so that either adjuster can move elevation or windage to suit left or right handed shooters. Getting such sights vertical can be difficult, but, if the aiming mark lacks wings or cross bars, the adjusters can sometimes be used to visually align the unit. You first need to attach the mount and lower half of the ring(s) (which are usually integral) to 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. You can use sandbags or a padded vise to get the rifle fixed in a level position, place the scope into the open mounting rings and screw the top rings down so they exert just enough pressure to hold the sight in place. 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. Rotate the sight until it looks perpendicular, then tighten diagonally opposed screws one turn at a time to ensure even distribution of pressure on the sight. Turn the elevation adjuster all the way down, being careful not to turn once resistance is met, then turn 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 adjustments are now centered. If the sight’s red dot is now nowhere near the target, the chances are you have mounted the sight incorrectly.
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.
If you have a spotting scope it really cuts down on walking if you have a calibrated target. Just draw a grid of known dimensions on a sheet of paper or board.. 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. 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 getting the rifle to put its shots on the card You can accomplish this in a variety of ways. If you have a bolt action rifle, you can remove 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. AR15s lend themselfves to this technique as you can break the action and withdraw the bolt carrier, then clamp 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. You should see a hole appear in the card or a bullet impact near it. Once you have a shot on the card you can fire a 3 shot group.. Take the difference in height of each of the three shots from the target center, add them up and divide the result by 3, then do the same with the windage. If you want to be extra sure of your zero, fire a few more groups, add the results together and average them. 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 (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. Assume you need to sight in 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 distance you will actually shoot from, i.e. 30 meters. 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. If you are 87mm off to the left you divide 87 by 4.8 to arrive at 18.125 which rounds down to 18. You now turn the windage dial 18 clicks to the right and you are on target in azimuth. Repeat the process with the elevation and you’re zeroed. Fire another three round group to check the result. 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 round 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 have access to a 100 yard/meter range your next step is to get the rifle hitting 2 inches high at that distance. Do so by firing three three shot groups and again taking their mean variation and using the calculation. A click will move your point of impact twice as far at 200 yards/meters as it does at 100. Manufacturers often publish the ballistics for their loads in tables giving their trajectory with a 200 yard zero and wind deflection values in m.p.h. at 90 degrees. These figures are sometimes a tad optimistic, but will give you a good idea of what you can expect from your catridge in a rifle of similar barrel length.
Once you have zeroed your rifle you should check the zero regularly – certainly before any hunting trip or competition and after the rifle takes any hard knock or is in storage for over six months.
Copyright Chris Pieterman 2010
Sighting In Your Rifle Scope In 4 shots or Less!