Useful Info

Article Index

The RifleScope - By Ian Kenney - Courtesy of The Shooting Voodoo

Finding Mechanical Zero on your Scope and Why it's Important

Courtesy of The ShootingVoodoo
Link to article on

The Riflescope

By Ian Kenney

In this article I will take a more detailed look at a modern, variable powered riflescope and what goes on inside as you start turning knobs and power rings.The illustrations depicted in this article will be pretty basic though because the actual design and location of some of the internal components will vary from manufacturer to manufacturer. Despite this I hope that at the end of this article you will have a better understanding of how a variable powered riflescope functions and better appreciate the advantages this piece of technology can offer.

First lets go through the basic nomenclature of a modern variable powered riflescope both inside and out.

On the Outside:

Descriptive photo of a rifle scope with parts labeled - outside.

On The Inside:

Cross section diagram of the inside of a rifle scope, with parts labeled.

What Do The Numbers Mean?

The meaning to all of the numbers gets a lot of people confused and in reality the answer is pretty simple. If you receive a scope that has 3-12X50, those numbers tell the user several things. The first two numbers represent the magnification range with three power being the low end and twelve power being the high end. The next number is the objective lens diameter, which in this case is 50mm. It's important to note that this isn't the outer diameter of the objective bell; it is only the diameter of the objective lens. It is important to understand the meaning of these numbers since it can be handy information when it comes time to select a scope or choose the proper mounting solutions. Failing to understand it can lead to purchasing the wrong scope for the job or rings that are the incorrect height.

The Main Tube

The main tube for a riflescope can either be a one or two-piece design, typically made out of high quality aluminum, however steel and titanium have been used as well. The main tube primarily contains and protects the major components of the riflescope in addition to providing a surface area for the scope rings to attach to. The finish most often used for the main tube and other external components is a Type II or Type III hard anondizing that can be just about any color specified by the manufacturer, however black is most common.

The Reticle

Reticles come in all manner of shapes and sizes so that the shooter can choose the one that will best fit his or her needs and situation. Some are simple crosshairs while others are more complicated with additional dots and/or hash marks that can be used for a variety of purposes. The distance between the dots and/or hashes will also depend on what system the reticle was designed around, being either milliradian or minute of angle-based. There are a few examples of both kinds shown below from a sampling of manufacturers that make quality riflescopes.

Line drawings of mildot-style reticlesLine drawing of mil-based reticles using hash marksLine drawing of MOA-based reticles

It is interesting to note that early riflescopes had simple crosshair-type reticles that were made of spider web. This was used because at the time the spider web was many times stronger than the wire available at the time, so it could withstand the recoil from heavy recoiling rifles. As time went on, stronger types of wire started to replace spider web as the material of choice when making scope reticles. While wire reticles are still used in some riflescopes, reticles etched in glass are common in many of the riflescopes used by military, law enforcement, and civilian shooters. Etched glass reticles offer a wider array of reticle styles and illumination options that were once just not possible. The reticle is on a flat piece of glass that is installed and epoxied into a reticle cell, which is in turn attached to the erector later on.

Adjusting the Diopter / Focusing The Reticle

Properly adjusting the diopter on a riflescope is something that many shooters, particularly new shooters, misunderstand the importance of. I've seen and heard many shooters claim that their scope was broken because they couldn't get the target image in focus by turning the ocular. In reality though, their scope is probably fine, it's just that they don't fully understand how the scope works. By turning the ocular, or ocular assembly, to the right or left, the shooter is increasing or decreasing the distance from ocular to the reticle, in turn focusing it to their eye, nothing more. Some people also mistakenly turn the ocular all the way in when adjusting the diopter and calling it good because the reticle looks sharp, when it's really not optimal. The user should only adjust the diopter to the point the reticle is nice and crisp against a light background, even when turning away and looking back through the scope. Also keep in mind that the diopter setting will be different from person to person since no two eyes, even in the same skull, are the same.

First Focal Plane and Second Focal Plane Reticles

Your typical, modern-day variable powered riflescope will have the reticle installed in one of two ways; it will either be located in the first or second focal plane. For most shooters the choice between first and second focal plane is based on cost, how they will be using the optics, and pure personal preference. In general though a scope with a first focal plane reticle will cost more than a one with a second focal plane reticle, even if they are the same magnification level. The reason for this is that designing and producing a scope with a first focal plane reticle is far more expensive and time consuming.

I'd like to first talk about reticles that are located in the first focal plane and what that means for the shooter. It wasn't but a few years ago that most of the scopes sold in the US that had first focal plane reticles came from European manufacturers.However, that is beginning to change with more and more domestic companies making the switch from SFP to FFP on some of their models. This is starting to bring the cost down on some models and open up the possibilities for many shooters to have an optic that is extremely versatile.

When I talk about first focal plane and second focal plane what I'm really getting at is the location of the reticle cell on the erector and inside the main tube. In first focal plane optics the reticle cell is located at the front of the erector and forward of the lenses in the erector that help control the magnification level. This means that as the magnification level changes the reticle will appear to get bigger and smaller from the shooter's perspective. This is where many shooters misunderstand and even start to dislike FFP reticles because they say that the reticle covers too much on high power and gets too small on low power. Now while the reticle does appear to be change size it is in fact maintaining its size in relation to the target. This means that if the target is five mils tall on high power then it will still be five mils tall on low power. Since the sub tension of the reticle doesn't change based on the magnification, all of your range estimation, trajectory compensation, and leads can be done on whichever magnification level is desired. This gives the tactical shooter a big advantage in the field, in competition, or when their life is on the line.

Illustration of the reticle cell, erector tube, and erector spring.An example of what it is like to look at a target on high power and then low power with a first focal plane reticle. Notice how the reticle still covers the same amount of the target despite the magnification change.

Probably the most common arrangement for a variable powered riflescope is to have the reticle installed in the second focal plane. This arrangement can be found from nearly every domestic optics company in nearly every price bracket. This set up is relatively cheap to design and easy to produce when compared to optics with a first focal plane reticle. Unlike the first focal plane reticle the reticle cell is installed at the end of the erector tube, close to the magnification ring. This places it behind the lenses that control the magnification level meaning that as the magnification level changes, the reticle will appear to stay the same size from the shooter's perspective. In reality though, as the target image gets smaller the reticle is covering up more of the target since it isn't changing in relation to the target. What this also means is that normally accurate ranging, hold-overs, and leads can only be done on one magnification setting without some sort of conversion.

Illistration of the erector tube and reticle cell (sfp)Illustration - this is an example of what it is like to look at a target on high power and then on a lower power with a second focal plane reticle. Even though the target is smaller the reticle appears to be the same size as on high power.

Both FFP and SFP arrangements have their advantages and disadvantages it is up to the end user's preferences, needs, and checkbook to decide which is best.

The Erector Assembly

One of the more complicated components of the riflescope it has numerous parts that are vital to the function and operation of the riflescope. This tube shaped assembly houses the reticle cell and a group of lenses that help control the magnification level of the scope. If the scope is equipped with an illuminated reticle, often times small LED lights and wires will be attached to the erector as well so it can be linked to the illumination knob. There is also the erector spring, which pushes against the erector to apply force against the two contact points for the elevation and windage to ensure consistent travel. Not only that but the erector also has to pivot smoothly at a point near the end of the erector so that it can actually move when pushed by the spring and or the elevation and windage knobs. Now that is a lot of stuff to be going on inside of the scope and hopefully this section will shed some light on exactly how it works.

Changing Magnification

When the user grasps the magnification ring and turns it to select a higher or lower power, not much thought is given to what goes inside to make that all happen. As the user is turning the magnification ring, it is connected to a portion of the erector that turns along with it and moves a pair of lenses inside of a scroll. The position of these lenses inside the erector determines how magnified the target will be from the shooter's perspective within the magnification range of the scope. As the lenses come closer together towards the front of the erector tube, the more magnified the target is to the shooter.

Illustration - Approximate position of the internal lenses with the scope on high power.

The more spread out and farther away from the objective lens they become, the lower the magnification level.

Making Adjustments

When the user turns the elevation or windage knob there are several parts that work together in order to make the bullet reach a certain distance and hit a specific target. Generally almost all tactical scopes have an elevation and windage assembly that the knobs slip over and attaches to. These assemblies contain numerous parts but mainly they house the click element and a spindle, typically made out of the brass, which protrudes down into the scope.

Illustration - The elevation assembly houses the click element and spindle. The elevation usually attaches to this with set screws. This threaded spindle is the part that actually makes contact with the erector to move it up and down.

The brass spindle will be tapped for a certain thread pitch that is based on the manufacturer's desired click value for the scope. This means that the thread pitch for a scope with ¼ MOA adjustments will quite different than one with 0.1 MRAD adjustments. Now, the click element that is also contained inside of the assembly helps to ensure that the spindle only moves an incremental amount to correspond with the desired click value. The most common click values in the tactical scope market are the aforementioned 1/4 MOA and 0.1 MRAD click values. Now what do those two values mean? Well, for every one click of the knob on a scope with 1/4 MOA click values, the point of aim moves 1/4" at 100 yards, or .26" for the True MOA crowd. The same pretty goes for 0.1 MRAD click values as well, one click of the knob moves the point of aim .36" at 100 yards, or .1 milliradian. The milliradian click values have been gaining immense popularity since no conversion is really necessary to go from what you see in the reticle to what you have to dial in for a correction. Now, if either part of the click element or brass spindle is assembled incorrectly at the factory, the scopes adjustments will not work out to what they are supposed to.

Now moving farther down to the erector….if the user is looking through the riflescope while turning the elevation or windage knob, they may notice that the reticle appears to move in the opposite direction from what they think it should. The reason for this is that the image that reaches the erector is actually upside down. When the user dials in more elevation, the erector is pushed upwards by the erector spring, pivoting at the rear of the erector on a sort of gimbal or ball-and-socket joint, making it appear to go down on the target image.

<Illustration - When more elevation is dialed into the scope the erector actually moves upward inside the tube, pivoting at the rear of the scope near the magnification ring.

The same goes for when less elevation is dialed into the scope, the reticle looks like it is going up when in fact it is being pushed down by the elevation spindle.

Illustration- When less elevation is dialed into the scope the erector actually moves downward inside the tube, compressing the erector spring.

Looking from the ocular through the erector: Illustration: Erector Tube, Erector Spring, Windage Assembly.

The erector spring really makes all of this possible since it has to be able to push the erector up into the spindles to ensure everything tracks as it is supposed to throughout the advertised range of adjustment. If it is too weak it will not be able to push it all the way up or to the side, resulting in the user experiencing “ghost clicks” towards the upper end of the travel range. This means that the user can turn the elevation or windage knob but no change in impact will occur, which can mean bad news for long range shooters. Sometimes a poor quality spring is to blame for this issue and sometimes it's a result of being overly compressed for an extended period of time. Something that I've always found interesting was that only a few millimeters of travel was necessary to reach the extreme limits of the scopes elevation and windage travel. For example if you were shooting to about 1000 yards and dialed in the elevation for the shot, the erector inside of the scope may have only moved about a millimeter depending on the cartridge that's being used.

Adjusting For Parallax

Parallax, to some degree, is present in almost all magnified optics and comes from the fact that the target image and the reticle are not focused onto the same focal plane. In optics with a lower magnification this does not present much of a problem but as you go higher in magnification some sort of compensation is required.

There are three ways to deal with parallax or compensate for it, have a scope with a fixed parallax setting, have an adjustable objective, or have a side focus knob. Fixed parallax scopes fix the focus lens in place and adjust the position of the objective lens in or out until the image is parallax free at a certain distance. Adjustable objectives also have the focus lens fixed in place but the objective assembly moves in and out by means of an outer ring on the objective bell. Scopes with a side parallax knobs deal with parallax in a different way by moving the focus lens instead of the objective. The focus lens is attached to the parallax knob by means of an arm that's in turn screwed to a slider riding inside of the parallax knob. As the parallax knob is turned, the slider moves inside of a scroll, pushing or pulling the focus lens, and allowing the image to become parallax free. Typically tactical riflescopes are able to compensate for parallax from 50 yards, or meters, to infinity, although some can go down as low as 10 meters.

Illustration - AS the parallax knob is turned towards the infinity setting the focus lens is moved towards the objective. When it is turned in the opposite direction for closer objects, the focus lens moves away from the objective.

Another interesting fact is that for most tactical scopes with side focus knobs, getting focused from 50 yards to infinity only requires moving the focus lens about a quarter of an inch.

In The End

What I've just described above was how your basic tactical riflescope works on the inside, as you would normally use it to make corrections and focus the target. Keep in mind that there can be ten or more actual lenses in the scope and upwards of a couple hundred individual parts depending on the brand and features. So you can see why some of the illustrations were pretty basic since it gets a little more complicated though when you start talking about zero-stops, illumination knobs, and other features that come on many high scopes out on the market. Hopefully though, this article has given you a better understanding of what is going inside your average tactical riflescope.

Mechanical Zero
And Why it's Important

By Scott Powers

Mechanical Zero Article Courtesy of SniperCountry and Scott Powers
To See the Actual Source Article Click HERE

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.