Zeroing a red dot or holographic sight is not a lot different from 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 have access to a boresighter and don’t have access to a formal centerfire rifle range.
You first need to make your weapon 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. Next ensure that your red dot scope is mounted perpendicular to the bore of your firearm. Many sights, for example the Eotech 553.A65 (United States Special Operations Command’s SU-231/PEQ (Model 553)), have mounts that align the sight automatically. Tube format sights can be turned in their mounts so either adjuster can change elevation or windage, making the unit ambidextrous. It can be difficult to get them vertical, but the adjusters can serve as a visual reference. You first need to attach the mount and lower half of the ring(s) (which are usually integral) to the rifle. In deciding where to place the mounts you also need to consider other items you may want to mount there including a magnifier or night vision system. Wedge the rifle between sandbags or cradle it in a padded vise and level it, then position the sight on the open mounting rings and screw down the top rings so they just gently hold the sight. Eye relief (important with magnifying reflex sights) can also be fine tuned 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, carefully counting the clicks as you turn. Turn the adjuster back down by half that number, then repeat the process with the windage dial. 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 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. All you need to do is draw a grid of known dimensions on a piece of card. A one inch grid works great with any sight – even the metric calibrated ones. 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 sights employing 2 MoA dots, your aiming mark needs to be 4 inch diameter for 100 yard shooting and 1 inch at 25 yards. You need to make up a half dozen or so of these targets, each with at least five aiming marks on as spares are always useful and you can always save them for next time. Try to get to the range on a calm day and try to avoid the hotter hours to prevent mirage becoming a problem. It will help to have some binoculars, a notepad and something to write with and a pocket calculator with you!
You first need to ensure your shots are on the target card. It doesn’t take specialized 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 (or another divisor of 100 yards/meters) away. You can now use the adjusters to get the red dot superimposed on the target. With the AR15 this is accomplished by removing the bolt carrier from the upper and clamping the barrel in a padded vise. This works well with many military semiautos, but will not work with Garands, M1As etc. or with most lever or pump 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 an odd way. The Trijicon TA31RCO-M4CP (the USMC’s AN/PVQ-31B Rifle Combat Optic for the M4 carbine) for example has 1/3″at 100 yards clicks and the Aimpoint Comp M4S (the new US Army M68 CCO) has them move the MPI 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 CompM4s 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. If you are 87mm off to the left you divide 87 by 4.8 to arrive at 18.125 which rounds down to 18. That is the number of clicks you need to dial in to the right to get on target in windage. Follow the same procedure with your elevation and you’ll be on target. Shoot another three round group to verify the result. This may sound onerous but it will soon become second nature and it will save you a lot of ammunition and frustration.
You may have iron sights to co-witness. Do not bother trying to get them in line with your red dot – both sights can be perfectly sighted in but appear misaligned because their axes are different but parallel. You now need to check your zero at a greater distance. If your red dot sight is low mounted (i.e. not on top of an M16 carrying handle) and is chambered for a high velocity round like the .223 or .308, a 25 yard zero will have your shots hit the target at 200 yards and be 2 inches high at 100 and 6-8 inches low at 300. If you can’t sight in at more than 100 yards where you live, your next task is to get the rifle hitting 2 inches high at that distance. Repeat the process outlined above. A click will move your point of impact twice as far at 200 yards/meters as it does at 100. Manufacturers will normally publish the ballistics of their individual loads, providing 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.
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