Portions of this article appear in another article by yours truly. Please forgive the redundancy but the topic seemed worth elaborating upon. If some of the following article appears familiar it is because portions appears in my article on scope mount selections.
When you purchase a telescopic sight, it comes with a specification based on internal adjustment. It is said to have 60 moa, 85 moa, 100 moa or even 120 moa of internal adjustment depending on the model. In practical terms, this means that at 100 yards, for instance, a 60 moa scope can traverse or elevate 60 inches from one end of its adjustment range to the other. Respectively, a 120 moa scope can traverse or elevate for 120 inches at 100 yards and an 85 moa will traverse 85 moa. Pretty basic stuff. This of course translates on down the line as range increases, which is what the term MOA (Minute of Angle) is all about. A 1 moa change in the scope at 100 yards represents a one inch change in drift or elevation. That same click equals 10 inches at 1000 yards.
Upon buying a new scope, the first thing you should do is verify the manufacturer's claim of internal adjustment. Doing so has several purposes.
Extreme windage adjustment is not particularly useful in the real world as anything over 20 mph or more can prove most difficult to negotiate in terms of hits on target at long range, particularly if the wind is shifting and gusting. But up to that point having a scope that is capable of handling such a drastic windage change is important. Still, the point here is that anything over an 15 moa adjustment at 1000 yards is going to be a tuff goal to hit and I can think of few tactical scopes, or even hunting scopes, that are not equipped to give more than enough adjustment in terms of windage. The most I have ever had to adjust windage at any range on any scope has probably been in the 10 to 15 moa range. Beyond that, at 1000 yards it's a real guessing game. Doable? Yep. But you will NEVER use the available 50 moa of right or left windage! Least not outside a tornado!
Elevation on the other hand is paramount. While you won't be cranking on the windage turret much, in relative terms, you will be making full revolutions on the elevation turret if you have any intent on shooting over a few hundred yards. Because of this, any scope, at least TACTICAL scope, with less than 50 moa is to be avoided if you plan on shooting over 600 to 700 yards. For details go read my Base Selection article which explains some of the ins and outs of that issue without actually being a scope selection article. Hmm... maybe in the future. The article you are now reading deals more with what you should do once you have your scope selected.
Upon removing the scope from the box and making sure its all there and nothing nasty is plastered on the inside of the glass (the horror stories I could tell), you want to check the sight's Mechanical Zero. This is an easy task, although sometimes tedious. Doing so will verify the manufacturer's claims of how much adjustability is built into the scope, show you any initial tracking problems and familiarize you with the feel of the turrets and how they "click" in.
Prior to zeroing your rifle it is important to take note of the actual mechanical zero of the sight because this allows you to one, have a solid starting datum and two, shows you just what the sight and rifle are capable of in REAL terms. Determining the mechanical zero is important because it tells you what the actual total internal adjustment is in your bright new toy. I am currently reviewing a scope that is claimed to have 100 moa of internal adjustment. The actual count is 115.5 moa, which is great. In this case the manufacturer erred on the conservative side in their claims. I have had, in another instance, a manufacturer claim 60 moa when the scope was only capable of 54 moa. Dialing the turrets and counting clicks will make plainly obvious the truth of what you hold in your hand and this data should be logged into your data book. If you are serious about your shooting, knowing everything you can about your system is just part of the game. In this case, knowing the full range of adjustment and the mechanical zero in turn lets you know just how far you will be able to shoot with that particular sight, based on what internal adjustment is left after your actual zero is achieved.
Example: Your sight has 60 moa of total adjustment. You have spun the dials and proven to yourself that you actually have 59 moa total. Your mechanical zero would be set at 29.5 moa from either side of the extreme adjustment range. Remember this. It will be useful later as explained in the article.
To find the mechanical zero of your telescopic sight, carefully turn one turret all the way to its stop. Do NOT force it once it lets you know that it does not want to turn any further. In other words, as you approach either extreme end of the range, slow down and carefully turn the turret so as not to force it past the last real click and into a damaging movement beyond. You can bend things that way! Now, turn the turret back in the opposite direction while counting clicks. Take care in counting clicks so you do not skip any or jump a series of clicks. Patience my friend, Patience. Go until you come to the stop at the other end of the adjustment range. Divide the total number of clicks by two and that represents the mechanical zero of the scope or sight. You may want to verify it by clicking back in the other direction and averaging the difference if any is present. Do this for both windage and elevation. Then set the elevation and windage turrets to reflect that number. Write the numbers down in your log.
On a scope represented to have 100 moa of internal adjustment, you should come darn close to 100 moa represented by what-ever incremental division is listed on the turrets. On a 1/4 moa scope, you are in for a lot of clicking as FOUR clicks equals ONE moa. So, to set the scope at its mechanical zero, from one extreme of the internal adjustment range, click BACK half the number of clicks. On this hypothetical 100 moa scope, you would need to go back 50 moa, or in terms of actual clicks, 200. Perform this operation with EVERY new scope you buy or with every iron sighted rifle you own that allows adjustment (like the AR15 for instance). While you are at it don't forget to take note of how smooth the turret turns, if the clicks feel solid or mushy, or if they bounce past an indented click too easily. The process is building muscle memory for you, so that the next time you are dialing in a range for real, say in a tactical match or shooting school, or hunting deer miles from any KD range, you won't be surprised by what is going on at your fingertips. That is why all this so important. But there is more.
In an ideal world (it obviously does not exist) you would be able to zero your rifle and scope combination at 100 yards at exactly the mechanical zero of the telescopic sight, or, better still, "below" that mechanical zero so that you had more UP elevation remaining than DOWN elevation. The odds of this are slim of course and no two rifles zero exactly the same way. However, you will occasionally find a combination that gets to with in 3 to 10 moa of mechanical zero either way. So using a typical scenario to illustrate this, let's examine it in more depth. Again, windage is least important as there is plenty to play with. There is one issue related to windage, but its for another day.
So... You have a .308 caliber brand X rifle and the factory did not totally screw the pooch and mount your barrel off at some odd angle that makes you feel drunk each time you look down the receiver. Your scope mounting holes are actually drilled fairly square to the center line of the bore for a change and when you bore sight the rifle you find that, lo and behold, you are only a few moa above or below the mechanical zero of the scope. We'll say 5 above for the sake of argument, but you might luck out and have much less "wastage".
On a scope with 100 moa of internal adjustment, you should have a mechanical zero of 50 moa or very close. Dial in the scope to that actual mechanical zero and reset your turrets to read ZERO. Mount the scope to the rifle, pack up your gear and go hit the range. Bore-sight the rifle at 100 yards by turning the turrets till what you see in through your rifle bore is what you see in the scope. DO NOT reset your turret caps to Zero. We are just trying to get on paper. Fire whatever series of rounds you need to actually zero the rifle for 100 yards. Once you have achieved this actual zero, take note of how far off this real zero is from your scope's mechanical zero. Write this down in your log. Again, the windage is not particularly relevant because you are never going to use it up. But the elevation is very important. If trying to reach maximum range, you can eat up all the internal adjustment in a scope.
Now, having shot your rifle and finding that to achieve your 100 yard zero, the rifle took 5 moa from mechanical zero in elevation, you can reset your turrets to the TRUE zero. From this point on, that is the only real zero you will worry about, but knowing where the scope started provides you with some important information.
In this scenario, you had to use up that 5 moa of internal elevation adjustment and you now only have 45 moa of UP adjustment left before hitting the stops. No big deal. With your .308 and 175 grain match bullets, you will easily be able to reach 1000 yards or so.
The story becomes a bit more bleak if your scope only has 60 moa of internal adjustment and your game is to reach 1000 yards. This scope would have a mechanical zero of 30 moa. Your 100 yard zero just used up 5 moa and sadly, it takes 41+/- moa to reach 1000 yards with a typical .308 caliber rifle from a 100 yard zero. You are now only left with 25 moa of UP elevation, or in real terms, about 740 yards reach, give or take. Still not bad unless you really need to reach 1000 yards for competition or tactical purposes. You can live with this and go home. Or not.
Here is where knowing the difference between your mechanical zero and your actual zero comes in real handy. Armed with this information, you can now assess whether you want to have your scope base shimmed, whether you want to go to a tapered base, or whether you need do nothing at all. As they say, knowledge is power and power is money: knowing what you know now, you can either SPEND more of it, or live with the current situation. It is also one more thing noted down in your log, which can be used for future reference. This works for both scoped rifles and those equipped with iron sights. Knowing your mechanical zero will tell you much about what options you have left. If you are trying to achieve a specific goal like 1000 yard shooting, it instantly lets you know if there is work to be done. It's a very simple thing in fact. One often overlooked. But one well worth the small effort put into it.