Zeroing a red dot or holographic sight is similar to zeroing a riflescope. Sighting in a riflescope without following a process is easy – if you have unlimited ammo and patience. This method is based on the assumption that you would rather not waste ammunition, don’t have a boresighter and don’t have a formal shooting range near where you live
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.
A calibrated target will help, especially if you have a spotting scope. Next, draw a grid of known dimensions on a piece of card or scrap plywood. A one inch grid will work with any red dot sight. The lines should be easy to see through your spotting scope out to 100 yards. The aiming marks should be large enough to accommodate 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. Make up a few of these targets, each with at least five aiming marks on each to have a few spares handy. It’s best if you go to the range on a on a day when neither wind nor mirage will be a problem – the latter can be avoided by going early. Whilst not essential, a notepad and pen, some binoculars and a pocket calculator could all come in handy (unless your mental arithmetic is very good!).
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.
Many red dot sights are calibrated in a non traditional way (i.e. not the 1/4 or 1/8 inch at a hundred yards most sportsmen are familiar with). The Trijicon TA31RCO (the USMC’s AN/PVQ-31B Rifle Combat Optic for the M4 carbine) for example utilizes on third of an inch at 100 yards clicks while with the Aimpoint Comp M4S (the new US Army M68 CCO) they mean point of impact by 16mm at 100 meters. To make matters worse, you could only have access to a range that makes use of natural terrain features and is thus some odd distance. Assume you need to sight in your CCO 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. So you start out with 16 and divide it by 100 which gives you .16, then multiply that by your zeroing range distance of 30 meters to give 4.8, which is how far each click will move the center of your group in mm at 30 meters. Let’s say your groups are 87mm off to the left. You divide 87 by 4.8 which arrives at 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 sighted in. Fire another three round group to check the result. This may seem a lot of work, but it means you use the minimum ammunition to get on target and the technique will work at any distance.
Now may be a good time to co-witness your iron sights. There is no need to make them appear in line with your red dot – both sights can be perfectly zeroed on the target but appear misaligned relative to each other. If you can, you should now check the zero at a greater distance. If your red dot sight is low mounted (i.e. not on top of an M4 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 step 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. A click will move your MPI 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 cartridge 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.
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