Sighting In Red Dot Sights
Red dot and holographic sights are similar to scopes in zeroing in procedure. Anyone can zero a scope by trial and error, but without knowing what you are doing you could waste a lot of ammunition. This method is based on the assumption that you don’t want to waste ammo, don’t have a boresighter and don’t have a formal target range near where you live
It is vital to first ensure your weapon is safe. Rifles are most often made safe by pointing the muzzle in a safe direction, detaching the magazine (if present), drawing back the bolt and manually inspecting 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)), have mounts that align the sight 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 construction can often be rotated so that either adjuster can move elevation or azimuth to suit left or right handed shooters. 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. 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 sights adjusters are now centered. If the sight’s aiming mark is now nowhere near the target, it’s likely 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 be beneficial 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 Aimpoints using 2 MoA dots, your aiming mark should be 4 inch diameter for 100 yard shooting and 1 inch 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. 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 ensuring the rounds hit the target card. You can accomplish this in a variety of ways. If you have a bolt action rifle, you can take out the bolt, place 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.. 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. If you want to be extra sure of your zero, fire a few more groups, add the results together and average them. 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 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 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 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. 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 may sound onerous 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. Do not bother trying to get them 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. 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 your range only goes out to 100 yards/meters, zero your scope to be two inches high at that distance. Repeat the method outlined above. 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.
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
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