Barrel Vise
Zeroing Red Dot Sights
Zeroing in a holographic or red dot sight has a lot in common with sighting in a scope. Sighting in a scope without following a process is easy – if you have unlimited ammunition and patience. This method assumes you don’t have limitless ammo or own a boresighter and don’t live near to a formal centerfire rifle range.
The first thing to do is ensure that your firearm 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. 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 (United States Special Operations Command’s SU-231/PEQ (Model 553)), incorporate 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. First attach the mount(s) with the lower part of the ring(s) onto 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. 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. Wind the elevation knob/dial right down, stopping as soon as resistance is met, then wind it all the way up, counting the clicks as you do so. Now wind the adjustment back by half that number and perform the same procedure with the windage adjuster. The adjustments are now centered. In the unlikely event that the red dot is way off target, the odds are there is something wrong with the way the unit is mounted on the rifle.
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. All you need to do is draw a grid of known dimensions on a piece of card.. A one inch grid works very well with all the calibrations used on red dot sights. 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. 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. Be sure to have binoculars, pen and paper and a pocket calculator with you!
You start by getting the rifle to put its shots on the card You can accomplish this in a variety of ways. 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 use the adjusters to get the red dot superimposed 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.
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.. 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. 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.
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 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. If you are 87mm off to the left you divide 87 by 4.8 to arrive at 18.125 which rounds down to 18. You now turn 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 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 align them 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. 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 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 wise precaution.
Copyright Chris Pieterman 2010
Barrel Installation with Action Wrench and Barrel Vise
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