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Definitive Guide to Mounting Scopes: The Scopes Trial

A PROPERLY MOUNTED SCOPE IS FIRST AND FOREMOST FOR ANY PRECISION SHOOTER. LEARN THE BEST PRACTICES PLUS SOME TIPS AND TRICKS FROM THE EXPERTS.

“Shooting is 70-percent science and 30-percent magic.” The type of shooter you are dictates which elements end up in which category. Scope mounting is one of those elements.

Let’s imagine you have a big rifle match or a hunt coming up in a few days. You’ve been preparing your gear and equipment for weeks and have multiple trips to the range. 

You feel fully comfortable with your ballistic data and your bullet drop at a multitude of ranges and under various conditions.

Your financial — and perhaps even more important, time — investment to this point is what we can call substantial. 

If you were asked if you were comfortable removing your scope from your rifle right now, how would you feel? Would you be slightly annoyed, or would you be losing your mind? There’s so much involved in precision shooting that some elements tend to get put in the “magic” category. 

People don’t like to screw around with magic. They are aware of the fact that this thing exists, but they can’t fully quantify its existence and they certainly can’t explain why it happens. 

This is the “magic” part of shooting. And it’s different for each shooter, but it’s what is most often blamed on a missed shot, a botched stage, or a ruined hunt — it’s that which we don’t understand. 

For those of you in which optics fall into that magical realm, allow me to try and pull the curtain back enough to expose the wizard.

OPTICAL THEORY: ABBREVIATED

To break it down into easily digestible chunks, we can consider the optical paradigm as three distinct portions: What is happening in front of the scope, between the front(objective) and rear(ocular) lenses, and what happens behind it. 

Internal assemblies of a telescopic rifle scope
Internal assemblies of a telescopic rifle scope

Visible spectrum radiation (light) is gathered through the front of our scope and is relayed through a series of refractive lenses to be displayed as a two-dimensional image to our eye. In between those lenses, we have a reticle superimposed on this image via engraving on a lens that we use to relate this image to three-dimensional space by way of alignment. 

Since projectiles don’t yet travel in a straight line, we must manipulate an internal erector tube (that contains our reticle) to align our reticle with the optical image by using the elevation and windage screws. 

These screws impart tilt on the front of the erector tube, while the rear is fixed, essentially pointing the reticle at different parts of the image coming in from the objective lens. It may seem like a lot of movement when you crank the elevation turret a few times, but keep in mind that the angular change we are observing is mechanically quite minor. 

The 20 MOA it takes to get your round real far out there equates to only 1/3 of a degree in your scope tube. 

What we’re trying to accomplish is aligning the reticle at the point in two-dimensional space on our image where our bullet's trajectory will pass through at a given distance (zero). 

Once the alignment is complete in the scope, the image is projected out the rear via the ocular lens in a small area for the consumption of our eye, commonly referred to as the eye box (or eye relief). 

Depending on the attributes and design of your scope, this eye box will vary in not only size, but also distance from the ocular lens. 

When perusing the specs on an optics manufacturer’s website, you might have seen the term exit pupil used. This is the diameter of the image projected in the eye box at certain levels of magnification. It’s also important to note that the distance and size of the eye box will change depending on the level of magnification used. 

So, what is our big takeaway from this brief jaunt through optical theory? Alignment. 

Imagine a line drawn through the center of your scope going straight back to your eye. We’ll call this the optical axis. Everything going forward needs to take place along this axis, so we need to make sure we can consistently place our eye coincident, or along this line. 

We can position a scope just about anywhere on our rifle, and if we maintain the sanctity of this alignment, we can expect good results.

What’s the best way to go about all this? The same as the best way to eat an elephant.

One bite at a time.

PREPARING YOUR SCOPE FOR MOUNTING

Arguably the most important part of setting up a scope on a rifle is properly focusing your reticle, by way of the diopter adjustment on the ocular side of the scope. Most scopes will have either a ring at the rear or it may be the entire rear housing. 

If this is not done properly, every following step will be an uphill battle and will make focusing on marksmanship fundamentals exceedingly difficult. 

This first critical step can be done as soon as you take the scope out of the box, but it should take precedence before any other step. The procedure is relatively simple but is often overlooked for the simple fact that it isn’t widely understood. 

What you’re trying to do here is essentially focusing the reticle (not the image) to your eye. The good news is, once this is done, you won’t have to worry about adjusting it again unless your eyesight changes. 

Long Range Shooting
When adjusting your scope mounting position, make sure the scope is in line with your line of sight with a comfortable cheek weld

1. Start by adjusting the parallax adjustment (if your scope has one) to infinity and dial up to max magnification. If you’re using a LPVO (Low Power Variable Optic) with a starting magnification of 1X, this sequence should be done at 1X. 

2. Unlock the diopter lock ring if it has one and twist the diopter ring either in all the way, or out until the reticle is quite blurry. You can start at either end of this adjustment, but I pull it out as personal preference.

3. Point the scope at a lightly colored, featureless surface so that your eye is drawn to the reticle and nothing else. You can use a piece of white printer paper, a white wall, or even a cloudless sky.

4. Glance through the scope for no more than a second or two and make a note of the sharpness of the reticle. If any part of the reticle is blurry, discontinue observation through the scope and pull your head away. 

5. Make a bold adjustment of the diopter and reassess the reticle for sharpness. Continue this until the reticle is crisp as soon as your eye acquires it. Your eye will naturally want to zero in on the center of the crosshair, but don’t neglect the stadia lines leading out to the edges of your field of view. You’ll know you’re done when the entirety of the visible reticle is clear and crisp in the field of view upon first glance. 

Again, don’t linger — your eye is incredibly effective at focusing a slightly blurry image, which might work in the short term, but will fatigue your eye and make prolonged observation difficult and possibly even slightly painful after a little while. 

DON'T FORGET YOUR LPVO

If you’re adjusting an LPVO, your goal should be to adjust the diopter until both the reticle and the image are in focus, and the image doesn’t show any optical aberration, which would look like a fisheye lens.

Once you’re comfortable with the focus, screw the diopter lock ring down snug. If your scope doesn’t have a lock ring, it’d be worth it to make a mark on your scope with a paint pen or a piece of tape, so you know if it gets twisted out of focus during a movement or transportation. 

This adjustment essentially focuses your eye to the reticle in your scope. If you ever find yourself having a hard time focusing on your reticle, this is typically the culprit. 

Don’t get this confused with sight picture; however; while the diopter adjustment focuses your eye to your reticle, the parallax adjustment aligns the sight picture plane with the reticle plane.

Having a blurry reticle and a blurry sight image will drive you mad trying to bring them together without a properly focused diopter.

For the last step of setting up your optic, we need to find the limits of our eye box. The eye box comprises the focal sweet spot at a certain distance from the ocular lens of the optic and the lens of the eye. 

The size of the eye box for a given scope is based on the size of the objective lens and the current magnification setting. The largest part of this sweet spot is known as the exit pupil and is commonly described as the optimal eye relief. 

Incoming artificial light against this .25-inch graph paper shows the exit pupil sizes at 7x
Incoming artificial light against this .25-inch graph paper shows the exit pupil sizes at 7x

The diameter of this area gets larger or smaller as you go down or up in magnification. If the magnification potential is very high, like you’d find on some ELR or benchrest rigs, then the position of the head becomes more critical than when on a lower power. 

If you’ve ever peered through one of these big tubes in the 25 to 55x range, you’ll likely have noticed that the eye box has nearly no forgiveness, and it almost seems like the scope is sucking the light out of the image. 

Dividing the magnification by the objective lens size will yield the exit pupil diameter, so if you have a 56mm objective lens scope set on 35x power, your exit pupil diameter is 1.6mm. That’s 1/16 of an inch, or about the diameter of the lead in a No. 2 pencil. 

Incoming artificial light against this .25-inch graph paper shows the exit pupil sizes at 35X
Incoming artificial light against this .25-inch graph paper shows the exit pupil sizes at 35x

Determining where your optimal eye relief is dictates the placement of the tube on the gun. 

It’s a bit complicated to get an accurate measurement, but I like to take a rough estimation of my eye relief at max magnification before I go on. 

At the maximum magnification setting, the exit pupil will be the smallest — it’s not critical, but it helps when determining ring placement on the base and where the scope will generally sit in the rings. There are a few different ways, but the easiest way most people can probably do this is with a ball cap. 

Place your scope on a level surface, pointing at something light so the image is decently clear. With a ball cap on your head, move your head back and forth until you completely eliminate any scope shadow. Make a mental note of the relation of the end of the bill of your hat and the rearmost portion of the scope, or even better, if the bill extends over the top of the scope, you can mark the end with a piece of tape. It might not work for everyone, but most find it useful and time-saving during the process of scope placement in the rings.

FIT THE RIFLE TO YOUR BODY, NOT YOUR BODY TO THE RIFLE   

Another commonly overlooked or neglected step is adjusting your length of pull. 

This is kind of a tough one, because there are a lot of stocks out there that don’t organically allow for any adjustment for length. 

There’s a whole rabbit hole tangent of how exactly this is determined, but lucky for us, 13.5 to 14 inches seems to be the industry standard and it tends to work for most people. There are many thoughts and opinions about how length of pull is properly adjusted, but the canary in the coal mine comes down to two things: wrist orientation and bolt clearance. 

This AI stock offers an immense amount of adjustability
This Accuracy International stock offers an immense amount of adjustability

When your palm lays along the side of the palm swell parallel to the stock in a natural shooting grip, your wrist should be at a comfortable angle where no muscular input or strain occurs. 

If you have to angle your wrist to the point that your elbow is cocked out at an extreme angle, you’re too close. Likewise, if you can’t manipulate the bolt to its full rearward travel without having to pull your face out of the way, you might want to increase the length of your stock. 

With optics setup in mind, the correct length of pull will yield a comfortable, upright head position that feels like you could take a little siesta on the stock. If you have to crane your neck up and down the length of the stock to get a decent sight picture, you’re going to get sick of shooting pretty quick. 

With the length of our stock sorted (or as best as can be), we can move on to the vertical alignment of our line of sight. It’s important to tackle these one at a time because we want to adjust one variable at a time. If you are trying to set your scope position on the rifle, it makes it much easier if we only worry about moving our eye fore and aft, and not up/down and side to side at the same time. 

We can adjust our sightline one of two ways: either by adjusting the comb height of the stock or exchanging the rings out for shorter or taller versions. Unless you have a stash of scope rings in various heights, adjusting the comb is normally the easier of the two methods. If you have an adjustable cheek piece, this step will be relatively straightforward. If not, your only option is to build up your stock to the correct height. 

A proper shooting position should look and feel neutral
A proper shooting position should look and feel neutral. Notice the upright position of the head and neck, and the squareness of the chest to the rifle. You should avoid major head tilt, neck craning or “scrunching” of the neck and shoulders. All this will induce muscular fatigue and reduce shot-to-shot consistency. A properly fitted stock is essential for full optimization and utilization of a mounted optic. Photo by Carrie Jones.

The old go-to in this instance is an old military sleeping iso-mat and some 100-mph tape. It’s not pretty, but it works a treat. To aid in alignment, you can use your scope loosely mounted in rings or just the ring bases themselves. 

If you’re using just the bare rings, treat them like a set of open sights and try to align the level of your line of sight to the imaginary line drawn between the tops of the two halves. For a scope, just worry about eliminating scope shadow above and below until you feel comfortable with the height of your eye. Be sure to mount the rifle multiple times to ensure you’re getting consistent results.

If you’re trying to determine what ring height you need, stick to this general principle: Start with the lowest height that’ll allow for at least 0.100 inch or one-tenth of an inch gap between the bottom of the objective bell and the top of your barrel or rail. There are various calculators available online if you need help. 

NO MORE BUBBLES

Before attaching rings to the base, put a drop or two of lubricant against the mating surfaces of the rings. This does two things — it helps to prevent moisture and corrosion forming as well as allowing the rings to “settle” as you’re breaking the mount in. If your mounting base has horizontal slots, apply forward pressure to the ring base and snug the cross-bolts finger tight, as you might have to move the rings before you’re done. 

You want your ring bases in full contact with the rail slots toward the front of the rifle to help eliminate any movement under recoil (think Newton’s first law). 

At this point, take a lint-free rag or a cotton ball soaked in acetone and give the inside of your rings and the scope tube body a quick rubdown. You’ll find this makes it easier to slide the scope forward and back and will give you greater grip on the scope once you’ve tightened the rings down. 

While you’ve got the acetone out, fill a small cup with acetone and drop your ring screws in there to degrease. Lay your scope in the rings and get in position behind your rifle. 

Assume whatever position you plan on doing most of your shooting in, just to make it easier. Doing this over a carpeted or padded surface is preferred, as the scope will be laying in open rings and can fall out. 

If you have a general idea of how long the eye relief is from our previous foray with the hat, put a piece of tape on the stock to the side of where your eye is positioned with a good cheek weld, and then measure forward of that point as a starting position. Since the eye box is smallest at the highest magnification, it’s preferred to do this at full power. 

In a well-lit area, move your scope to a position where a full field of view is achieved, and you see no scope shadow. 

This stock was cut out to fit a field expedient cheek riser and is removable for barrel cleaning
This stock was cut out to fit a field expedient cheek riser and is removable for barrel cleaning. The stock pad covers the removable comb and adds cushion and storage for tools or equipment. Get creative!

If the rings restrict you from getting to this position, now is the time to move them around. Ideally you position rings above or as close as possible to the part of the base that attaches to the receiver for as much rigidity as possible. 

As long as your rings don’t butt up against the bulbous portion of the scope tube where the erector assembly lives, you’re good to go. Put a piece of tape around the scope just behind the rear ring to mark the position. Put the tops of the rings on and grab your screws out of the acetone and start threading them in. 

Snug them down to the point where the scope doesn’t move around, but you can still slide it back and forth with minimal effort. 

The most commonly misunderstood part of this entire process is scope leveling. Many believe if the reticle isn’t perfectly level to the rifle it’d somehow impede their ability to connect with the target. This couldn’t be further from the truth. 

Your scope (and therefore your reticle) should be level to the earth, not the rifle, as gravitational pull has the biggest effect to your trajectory. Yes, it can be visually comforting to have the reticle level with the orientation of your rifle, but it’s by no means critical for a successful long-range shot.

Your scope mounted at a 45-degree angle can still make hits at distance as long as the reticle is aligned with the pull of gravity, and you had a good dope chart. If you don’t believe it, look up examples of “chin guns” used in the ’90s in high-power silhouette competition. 

The main detriment to having your reticle not level with your scope base is the artificial consumption of windage that comes from the difference in attitude between the optical and bore axes. This, of course, requires the assumption that your barrel is absolutely straight, and your scope base, rings and scope all sit absolutely straight above the bore axis. Here’s a hint: They do not, and they never will. So, when leveling a scope, you can use a pretty simple method and not worry too much about it.  

Using a small machinist’s parallel bar, or any thin, rigid implement with parallel sides, slide it under the scope body and, using the flat underneath the elevation and windage housing and the top of the base, rotate the bar until both are in full contact with the sides of the bar. 

Machinist’s parallel bar used to square the top of the scope base to the flat bottom of the erector assembly bulb
A cheap, imported machinist’s parallel bar used to square the top of the scope base to the flat bottom of the erector assembly bulb. It’s precise and much faster than using multiple bubble levels.

At this point, a mechanical level has been achieved to the scope base and you can begin to tighten the ring screws. Arisaka Defense sells a kit that operates on this same principle.

It’s important to hold this bar in tension until you’ve tightened at least one ring fully to avoid any torque steer from the rings clamping down on the scope tube body. 

Tighten your scope ring screws incrementally using a crisscross pattern to evenly spread the tension over the tube.

After consulting the ring manufacturer’s specifications, torque the ring screws down while trying to keep the gap between the top and bottom of the rings even on both sides. 

Most manufacturers will tell you between 15- to 18-inch pounds. While a lot of people do it, you typically don’t need to apply any type of thread locker to these screws. While most manufacturers are pretty hush on the issue, some, like Vortex, advise against it. 

If you want to learn more about correct torque and the use of thread locker, take a look at my article “The Glue that Binds Us” in RECOIL Issue 50. With the rings torqued down, torque the cross-bolts to spec. 

loctite threadlocker

Check your setup by mounting the rifle in various positions and ensure everything is repeatable and you don’t have to “hunt” for proper eye relief when you put your face on the stock. If you’re satisfied, a quick swipe with a paint pen across your screws will allow a quick visual verification that they haven’t loosened.

LOOSE ROUNDS

Hopefully you’ve walked away from this with a better understanding of how your scope works and the proper way to attach it to your rifle. If you’re considering getting involved in the exciting world of long-range shooting, it’s highly recommended to use a scope level or cant indicator to ride on your scope tube. It’ll set you up for success by mitigating a huge variable in distance shooting. 

When I have the opportunity to teach scoped rifle shooting, the first thing I like to do is have the students remove their optics from the rifles. You can imagine the visible concern on their faces, because they thought they had lost their precious “data.” 

We go through proper rifle setup, the reasons behind each step, and by the time we get to putting the scopes back on, you can see the stress dissolve when they realize the rifle fits them better and their data didn’t change much, if at all. 

It does seem like a monumental task to carry out, but once we strip away the magic, it comes down to some pretty understandable science. 

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