Mounts Scopes

Sighting In Red Dot Sights

Red dot and holographic sights are similar to traditional riflescopes in zeroing in procedure. Anyone can zero a scope – the trick is to use as few rounds as possible. This method assumes you don’t have limitless ammo or access to a boresighter and don’t live near to a formal centerfire rifle range.

The first thing to do is make sure that your hunting arm is made 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 sight is mounted perpendicular to the bore of your rifle. Many sights, for example the Eotech 553 (United States Special Operations Command’s SU-231/PEQ (Model 553)), incorporate mounts that do this automatically. Tube format sights can be rotated in their mounts so either adjuster can alter elevation or windage, making the unit ambidextrous. 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 sight. 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. 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. 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 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 should have access to somewhere to shoot targets with a safe backstop. It should allow shooting at at least 100 yards/meters but 200 yards is preferable. 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 works very well with all the calibrations used on red dot sights. The lines should be visible through your spotting scope at 100 yards. 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. 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 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 ensuring the rounds hit the target card. It doesn’t take specialised equipment to do this. If you have a bolt action rifle, you can remove 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. 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. 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 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-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 CompM4s (the new US Army M68 CCO) has clicks that move the 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. 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. That is the number of clicks you need to make to the right to get on target in windage. 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 align them with your red dot – both sights can be perfectly zeroed on the target but appear misaligned relative to each other. You now need to check the result at a longer range. 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 cartridge 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 task is to get the rifle shooting 2 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 will normally publish the ballistics of their loads giving 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.

Copyright Chris Pieterman 2010

Mounting A Scope