Zeroing 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 ammunition or own a boresighter and don’t have access to a formal centerfire rifle range.
It is vital to first ensure your firearm 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. The red dot sight must be mounted level on the rifle or the rifle will be canted, ensuring your rifle will only be zeroed at one distance and will shoot wide at others. Many sights, for example the Eotech 553 (United States Special Operations Command’s SU-231/PEQ (Model 553)), have mounts that do this automatically. Tube format sights can be turned in their rings so either adjuster can alter elevation or windage, making the sight ambidextrous. It can be difficult to get such sights 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. 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 fine tuned at this stage by pushing the scope through the rings if clearance allows. 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 rotate 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 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 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. 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. 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 There are a few ways of doing 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 use the adjusters to get the red dot superimposed 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. This works well with many military semiautos, but will not work with Garands, M1As etc. or with most lever or pump actions.
The next best alternative is to put a card out at 25 yards/meters and fire a single shots from a rested position - shooting from sandbags is best. Do not use a bipod or rest the rifle directly on anything solid. Your shots should be on the card or close to 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. 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.
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 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 shift the mean point of impact by 16mm at 100 meters. To compound the problem, you could only have access to a range that makes use of natural terrain features and is thus some odd distance. 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 wind 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. Shoot another three round group to check your zero. This may sound tiresome 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 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 chambering 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. 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 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
Filed under: Rifles and Shotguns on January 26th, 2011 | No Comments »