It’s a scenario I’ve seen dozens of times: a frustrated shooter with spent cartridges all over the shooting bench and still no zero. Sighting in can be frustrating if not properly approached, but if done correctly, it’s a simple and painless process.
The first objective is to remove all variables possible. That means making sure the scope is properly mounted to the rifle, using a steady and solid rest to allow repeatable shots, and setting up a target large enough to record any shots fired. Once this is accomplished, check the scope for parallax by moving your eye back and forth behind the ocular lens and watching to see if the reticle moves across the target. If the crosshairs move, you have a parallax problem that will affect your ability to fire accurate shots. If your scope has adjustable parallax, make sure it's set to the proper distance.
Choose the right distance for zeroing
If you’re just checking zero on an existing rifle and scope combination that you've used before, there should be no problem recording shots when shooting at 100 yards. The odds are pretty good that your zero is either still right on or pretty close. This scenario is common if you're just re-checking your rifle at the beginning of the season. If you're mounting a new scope, you have no way to now how much the scope and rifle are out of alignment, so it's best to start your zeroing process at 25 yards.
Fire one shot and note the placement of that shot on the target. At this short 25-yard distance, you’re only getting a rough zero that will assure hitting paper at 100 yards, so it’s reasonable to make adjustments off that one shot. You aren't establishing absolute precision just yet.
Adjust the scope using the manufacturer’s values, remembering that you'll probably need to multiply the number of clicks by four because most scope adjustment clicks are calibrated for a 100-yard distance.
Many scopes are calibrated in minutes of angle (MOA) and move the point of impact 1/4-inch per click at 100 yards. That means that each click will only move your impact 1/16th of an inch at 25 yards, so adjust accordingly. If your scope adjustments are in milliradians (mils), then know that one mil equals about 3.6 inches at 100 yards and .8 inches at 25 yards. Many milliradian scopes use 0.1 mil-per-click adjustments, so it will take ten clicks to move impact .8 inches at a distance of 25 yards. These numbers are rounded off, but the value of error is much less than a click, and they’ve served just fine for me out to 1,000 yards. After making the adjustments, fire another shot and if you’re within an inch of center, you’re ready to move on to 100 yards.
At 100 yards, fire three shots and repeat the process, keeping in mind that the number of clicks required will change by a factor of four. The more accurate the rifle, the easier it is to get a precise zero. No matter how many rounds you fire, you can’t get a zero that’s closer than the accuracy capability of the rifle. After making the appropriate adjustments, fire another three shots. If your measurements and adjustments are correct and your scope has accurate adjustments, you should have a zero. If not, you likely have a problem with the scope.
How to zero with two shots
The above is the procedure for sighting in a rifle/scope/load with which you have no experience. If you have a rifle and load that you know is capable of accuracy under one MOA, the process can be much simpler. While shooting off any good rest works for making calculated adjustments, using a rest that holds the rifle in place can allow checking or correcting the zero in two shots.
After making the first shot, adjust the rest, so the scope is again centered on that first shot aiming point. The setup has to be solid enough so that you can move the turrets without moving the rifle. Once the crosshairs are centered, move the scope adjustment knobs to the bullet hole of the shot you just fired. If the rifle doesn’t move in this process, the next shot should be centered within the accuracy capability of the rifle.
This is an easy process that competitive shooters and outfitters have used for years, and if you do it right, it will work every time. Remember, it only works within the capability of the rifle, so I’d never even try it on a rifle capable of less than 1.5 MOA. With less accurate rifles firing more shots will give you more confidence that you have a good zero. Even on the most accurate rifles, it makes good sense to follow up with a couple more shots to make sure your zero is centered up correctly.
The two shot sight in method isn’t a replacement for a serious sighting in session where multiple shots are fired, and a trend is observed for finer adjustments. However, it’s a great way to get to that point without extensive shooting, guessing the number of clicks required, and lots of trial and error. It works remarkably well, but it is a shortcut, not an end result. Always remember that changing ammunition brands, bullet weights, or styles requires re-checking and most times re-adjusting the scope.
More on parallax and focus
Parallax is often confused with focus, but the two are completely different. Focus adjustments should be made at the ocular lens of the scope to clarify the appearance of the reticle itself. To properly focus a scope, look at a white or opaque surface and rotate the ocular lens adjustment until the reticle is sharply in focus. To make sure you have the best focus, look away at something else for a few seconds and look back a couple of times making finer adjustments if needed.
Proper parallax settings affect the clarity of the image and change with distance to the target. The marks on the parallax knob or objective bell are approximate and only for reference. Because they affect the alignment of the image with the reticle, they should be checked for best accuracy. To adjust parallax for a specific range, place the rifle on a solid rest and adjust until you can see the target through your scope. Move your head back and forth and up and down across the ocular lens. If the reticle seems to crawl across the target, the parallax is incorrect for that range, and you'll need to make additional adjustment.
Rifle scopes without external parallax adjustments usually come pre-set at the factory at 100 yards for centerfire scopes and 50 yards for rimfires. This is why centerfire scopes without external adjustments rarely work on rimfires. Even though they aren’t adjustable, it’s a good idea to check them before sighting in because sometimes scopes leave the factory improperly set.
Whatever method you use to establish or confirm your zero, be sure to eliminate as many variables as possible so that you can be sure your adjustments are correct.