Mounting a Scope

Sometimes it seems that nothing is more confusing to a new gun owner, and even some old hats, as installing a scope on a firearm. The sheer number of choices and options in optics alone are enough to write books about, and the ways to actually put the scope on the gun are even more complicated.

This article will attempt to clarify choices and make the novice gun owner’s job in choosing a way to mount a scope easier, while at the same time illuminate the myriad of options for any gun owner.

What You Need to Know (or Find Out)
When you start looking to mount a particular scope on your firearm you need to know certain things before you start shopping for mounting options. To begin with, you’ll need to know your scope’s objective diameter, usually the last and largest number in a scope’s specifications. This will be used to determine the clearance needed for your scope. The next important measurement you’ll need is the scope tube, and thus what ring diameter is required by your scope, usually 1″ or 30mm.

You need to know what mounting system your rifle is equipped with; receiver grooves, Picatinny rail, Weaver dovetail base, or nothing at all. The type of firearm you have and in some cases, what barrel type and contour, will also impact what mounting system you will need to use. A flat-top AR-15 has vastly different ring requirements than a Remington 700 with a tapered barrel.

The Basics
The most common setup used, and inquired about, for putting optics on a long gun is mounting a traditional magnified rifle scope to a traditional bolt action rifle. Let’s use this setup to explore the basics of putting a scope on a rifle. Traditionally the scope is held onto the rifle by “rings” which are clamped around the body of the scope. In turn the rings are attached to some sort of “base”, which are normally then attached to the rifles receiver or action.

To add to the confusion, there are many variations of all of these components. There are ways mount a scope that combine some or even all of these individual components. (An example would be mounting an EOtech HWS to an AR-15 rifle, where the scope has an integral mount that does not require rings, and the rifle has an integral rail that does not require a base.)

Bases
The base is the part of the system that allows the attachment of the scope rings to the rifle itself. Traditionally bases are attached to the receiver of a firearm by some sort of screw or bolt. Some rifles have built-in bases that are integral to the firearm.

Bases are usually specific to the make and model of firearm for which they are designed, and most are specific to the type of rings that can be used with them.

Rings
The Ring you select must be the same inner diameter of your scope’s tube diameter, which makes sense. The rings clamp onto the main tube of the scope, so they must be the same size. Most scopes are 1 inch in diameter and so are most rings, but there are many 30mm scopes and rings, and a smattering of other choices, such as 34mm. The other end of the ring attaches to the rifle’s base, so it too must be compatible with that particular base.

The important thing to remember is to match the rings with the scope and the base.

Ring Height
Scope rings come in varying heights to vary the distance from the scope’s centerline to the firearm’s base or receiver. The different ring heights allow you to mount scopes with different objective sizes and also to mount the scope in the proper place for a quick and proper cheekweld. Rings normally come in low, medium and high heights. There are extra low and extra high variations from some manufactures. Ring height is one of the most confusing options for new gun owners.

The traditional mantra is that scopes should be mounted as low as possible without the front of the scope, the objective bell housing, touching the barrel. Also bearing in mind the need to the back of the scope, the ocular bell or eyepiece, to clear the bolt handle. The thing most often ignored in this choosing scope ring height is the need for the scope’s height to match the shooter. When the rifle is shouldered quickly, the scope should be at the correct height to look through it, without any additional movement on the stock up or down to get a good view through the scope.

Remember though that there is no “perfect” ring height that would suit every person, as each person’s physiology is a little bit different. There is a standard ring height worth noting, as it gives an excellent starting point for 90% of gun owners.

In this illustration from UTG, Ring height is measured with “C”

Ring Height According to Leupold

• 50mm objectives will almost always use HIGH rings in a given style. In certain instances, such as with extremely heavy barrels or some makes of firearm, EXTRA HIGH rings may be necessary.
• 42-45mm objectives will almost always use MEDIUM rings in a given style. In certain instances, 45mm scopes may require HIGH rings.
• 40mm objectives will almost always have enough clearance with LOW rings in a given style, though MEDIUM rings will give slightly more clearance, particularly when using a barrel with a thicker shank portion or a heavier contour.
• 28-36mm objectives will almost always use LOW rings in a given style. Again, in certain instances of a heavy barrel or heavy shank portion of a custom barrel, MEDUIM rings may have to be used, but LOW rings will almost always suffice.
• 20-24mm objectives will almost always be able to use LOW rings, but in some cases may also use EXTRA LOW rings. In this instance, bolt handle clearance of the eyepiecel will come into play more so than objective / barrel clearance and should be carefully considered.

No Leupold riflescope will fit into EXTRA LOW rings if using a one-piece base.

Ring Height According to Weaver

• Use low 1″ Rings for up to a 38mm Scope Objective
• Use medium 1″ Rings for up to a 40mm Scope Objective
• Use high 1″ Rings for up to a 44mm Scope Objective
• Use extra high 1″ Rings for up to a 50mm Scope Objective
• Use 30mm Low Rings for up to a 33mm Scope Objective
• Use 30mm high Rings for up to a 44mm Scope Objective

For a more in depth look at ring height click here to refer to our ring height article.

Now that we have discussed ring height standards, there are some notable exceptions: for the AR-15 and M-4 series of rifles with “flat top” receivers extra-high rings are the rule of thumb, but some applications use high rings. Also, for mounting a scope on a Harrington and Richardson Handi-Rifle or the NEF Pardner you will need at least medium and perhaps high rings clearance between the scope’s eyepiece and the rifle’s hammer.

Mounting Systems
Weaver
The most common scope mounting system is the Weaver system. The weaver system utilizes flat dovetail rails with crosswise slots found on everything from rifles to shotguns to handguns. The Weaver style bases are 7/8 inch wide and are designed to accept Weaver style rings. Most rings manufacturers make a Weaver style ring and/or rail. On Weaver style rings the bolt used to secure the ring runs underneath the web of the ring and fits into a corresponding crosswise slot in the Weaver base. This prevents any scope movement fore and aft under recoil or abuse. The bases can be found in one or two piece configurations and can be made of steel or aluminum. The Weaver style system allow the Weaver ring to be detached from the base with the scope still in the rings and reattached without any major loss of zero. The optic can be removed from the gun for maintenance, transportation or storage. The weaver system also allows for using different optics on one weapon or one optic on different weapons.

Picatinny and MIL-STD-1913
Picatinny rails are very similar to Weaver rails. The difference is a published military standard. The physical difference between Weaver rails and Picatinny rails are the width of the slots that are cut crosswise in the base, and the spacing of these slots. The slots in Picatinny bases are wider than slots in a Weaver base, and the spacing of these slots in a Picatinny rail is consistent and standardized. The Weaver style rail’s spacing of the cross slots are not standardized and the spacing is left up to the manufacturer. The recoil lugs on Picatinny rings are thicker to fit in the corresponding wider slots in the Picatinny bases. Weaver rings will fit on Picatinny bases, but Picatinny rings won’t fit on Weaver bases.

22 Rimfire Rings, Tip-Off Rings and 3/8″ Dovetail Rings
Most .22 rimfire rifles and airguns produced today have cuts running lengthwise in the top of the receiver to mount rings on. Some European .22s and air rifles have grooves that measure 11mm or 13mm. 3/8″ dovetail rings are clamped into the groove of these grooved receivers, these rings are also referred to as rimfire rings, .22 rings, and Weaver brands them Tip-Off rings. There are also rifle found with 3/8″ bases attached to the receiver with screws as a normal base would be, and you can buy 3/8″ bases to add to most firearms without them. Some rifles with a grooved receiver are drilled and tapped for Weaver style bases. We recommend using a Weaver style base in these cases as they offer more area for the rings to grab and are more secure and stable.

Redfield Style
The second most common mount type is the Redfield style. This type of mounting system, like the Weaver, has many clones and manufactures of bases and rings. The bases can be one or two pieces, and are known for their sleekness, and strength. Unlike Weaver style rings, Redfield style rings are not detachable from the rails system. The top half of the rings must be separated from the bottom half to remove your scope. The front ring in the Redfield system has a dovetail that turns into a corresponding slot in the front base, allowing the scope to pivot around this point. The rear ring in the Redfield system sits on the base and is held by two opposing windage screws tightened into the ring. The windage screws have a leading edge that fit a corresponding slot in the ring. By loosening out one screw and tightening the other, the ring moves right and left on the base, acting as an external adjustment. This adjustment allows you to zero the windage of your scope without using its internal adjustments.

Dual Dovetail (DD) Systems
Dual dovetail bases are the same as Redfield style, but instead of the windage screws holding the rear ring to the base, the rear ring is turned in just like the front. This system does not offer the windage adjustments that the standard Redfield style bases offer. This system is secure and a very strong system for heavy recoiling rifles and handguns.

Clamp-on Mounts
Clamp on mount such as made by B-Square, S&K, ATI and other manufacturers allow easy scope mounting on guns that aren’t drilled and tapped for scope mounts. These mounts typically do not require gunsmithing, and are easily removed without any modifications to the firearm. Older and military surplus firearms often benefit greatly with these style mounts.

Maximum Point Blank Range and the Battlesight Zero

The MPBR is the maximum range at which the bullet rise and drop stays within the vital area of your target. Anyone who has been in the U.S. Army or Marine Corps is familiar with a battlesight zero or improved battlesight zero (BZ0 or IBZ0). The concept for an MPBR or battlesight zero is pretty much the same: zero the rifle so that you get a point of aim that is effective over the longest range. The battlesight zero used by Marines when shooting the iron sight M16A2 is the 36/300 zero, meaning that the bullet will be on the sight line at 36 yards and again at 300 yards. The US Army uses what is referred to as an improved battlesight zero, which calibrates the rifle to be dead on at 50 and at 225 yards. The USMC also uses the 50/225 IBZ0 for M16A3 rifles equipped with Trijicon ACOG scopes.


The illustration above demonstrates how a battlesight zero works. The bullet is fired from the barrel and rises up to be exactly on the line of sight at 36 yards. It then continues to rise, topping out at 6"-7" depending on the round used and the barrel length of the rifle. It then descends until it is again exactly on the line of sight at 300 yards. This gives the Marine a good aiming point for a man sized target at any distance between 0 and just over 300 yards.

From the USMC manual:

If a rifle is zeroed for 300 yards, the bullet crosses the line of sight twice. It first crosses the line of sight on its upward path of trajectory at 36 yards, and again farther down range at 300 yards. Since a bullet crosses the line of sight at 36 yards and again at 300 yards when a rifle is zeroed, a rifle's zero may be established at a distance of 36 yards and the same zero will be effective at 300 yards. It is critical that a Marine fires tightly grouped shots directly on the point of aim when establishing a BZO at 36 yards because any error in shot placement at 36 yards will magnify as the bullet travels down range.

If the rifle is properly zeroed for 300 yards/meters, the trajectory (path of the bullet) will rise approximately 7 1/2 inches above the line of sight at a distance of approximately 175 yards/meters. At other distances, the strike of the bullet will be less than 7 1/2 inches above the point of aim. Only at 36 yards/30 meters and 300 yards/meters does the point of impact coincide with the point of aim. If only a portion of the target is visible (e.g., the head of an enemy soldier), the trajectory of the bullet may have to be taken into consideration when firing at a distance other than 300 yards/meters. If a Marine does not consider trajectory, he may shoot over the top of the target if the target is small and at a distance other than 300 yards/meters.

The 50/225 IBZ0 is useful as the bullet has much less rise at the midpoint of the trajectory. Its shorter effective range is more suited to urban and jungle warfare where visibility is limited and most engagements are at close range. The fact that the bullet rise is lower means that shots taken at ranges between 0 and 250 yards are much more accurate, with a bullet rise less than 2 inches at the midpoint of the trajectory.

The battlesight zero as a concept is very useful to hunters as well. When hunting deer, or any medium sized game, it is rare to know the exact distance that the quarry will be encountered at. Luckily, if your rifle is properly sighted in for its maximum point blank range (MPBR) you don't need to know the exact distance. While the ballistics vary from rifle to rifle, it is generally a simple matter using any number of online ballistic calculators to work out what the ideal zero for your rifle should be. The most critical calculation is your second zero. Based off of the size of the vital area of your target, you can compute the maximum rise and drop tolerable for your cartridge. Most white tail deer for example have a vital area that is generally 10 inches in diameter. Mule deer, elk, and moose have vital areas that are significantly larger. A large mule deer has a vital area around 12" in diameter, an average elk around 15", and a good sized moose nearly 21". A hit from a medium caliber rifle to this area will result in a quick kill. Therefore, if we are hunting white tailed deer, we can tolerate a maximum rise and drop of 5". Using this value, it is simple to calculate that the MPBR for a 180 grain Remington Core-Lokt .308 roundnose soft point cartridge in my trusty Remington 700 is 293 yards, with our second zero at 252 yards. With our rifle zeroed for these distances, we can be assured that a perfectly centered aim on a deer at any distance between 0 and nearly 300 yards will result in a hit in the vital area of our target.

The problem with zeroing your rifle for 293 yards in this case is that not many people have access to a 300 yard range. Not to worry, there are other ways to achieve the same zero for your hunting rifle. As it mentions in the USMC manual we referenced above, you can sight in your rifle at a closer range for the same result. In fact, if you have a good bench rest and a gridded target you can, with a little math, perfectly achieve a MPBR zero on your rifle at any range. Lets assume for this example that our range only has a 50 yard rifle range. We're shooting a Remington 700 chambered in .308 and plan to use the 180 grain Remington Core-Lokt mentioned above. By plugging in the information for that load, we can see that the bullet should hit 2.2" high at 50 yards (if you were at a 100 yard range, it would hit 4.45" high). Our first zero for this rifle and cartridge combination is actually just shy of 20 yards, and you can use that distance if that is the only range available at your local shooting gallery, but be aware that minute errors in measurement which may not be apparent at that close range will be magnified at longer distances, possibly throwing your shot off.

Remember: The Battlesight Zero and Improved Battlesight Zero discussed here only work on 5.56 M16 and AR-15 style rifles. You will need to find the maximum point blank range for your unique rifle, optic, and cartridge combination. Even differences such as the scope you have mounted on your particular rifle will change the MPBR and subsequent zero. Find the manufacturers information on your favorite rifle load, google up a ballistics calculator, and in just a few minutes after plugging in your date you'll have a good MPBR zero for your setup.

Sighting in Your Scoped Rifle

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When sighting in your rifle, you need to make sure that you begin with a good solid bench and an adjustable but solid rifle rest. Many different guns rests are on the market, from advanced fully adjustable machine rests, to rabbit ear bags and old style shot filled bags. Machine rests generally have, at a minimum, a front forend rest similar to The Rock Jr Shooting Rest. This forend rest is adjustable for elevation so that you can easily and precisely position the rifle. A rear bag is also helpful, even with some machine rests. Other more robust machine rests have fully adjustable front and rear rests, and some like the HySkore Dangerous Game Rest even include a remote trigger, enabling you to fire the rifle without touching it at all.

Looking down the bore at the target to obtain a bore sight.

Consistency is key when sighting in a rifle. Every shot should break exactly the same. You don't need to be fighting your rest or your bags when setting up for your sighting in shots. Whether you are using a machine rest or sand or shot filled bags, take the time to set up your rest properly so that when the time comes to fire the shot the only thing you are doing is squeezing the trigger. When properly setup on a bench rest prior to the shot, your rifle should be fully supported by the rest and able to remain perfectly centered on your target without any corrections from you the shooter.

Wind flags and pinwheels are very helpful when sighting in so that your shots are not thrown off by the wind. In the absence of wind flags, pay attention to vegetation such as trees, bushes, and grass to see how and when the wind is blowing. One thing you don't want is to have some shots fired when the wind is blowing while others are shot during lulls in the breeze. Spend some time analyzing the wind and make a conscious decision as to whether you will be firing when the wind is up or during the lulls. Ideally, you should shoot when the wind is down or non-existant. But if that is not possible, you can make accurate estimates of wind speed and direction and use that to adjust the zero on your scope to compensate for the wind. Keep an eye out next week for our article on reading the wind and mirage, where we'll go into detail on how to read and adjust for these conditions.

Choosing a good target for sighting in your rifle is critical to making the process painless and accurate. Our VisiShot Sight-in Paper Targets feature a grid for easy windage and elevation adjustments, as well as a high visibility background that makes it very easy to see where your shots are hitting the paper. Whatever target style you choose, having a one with a 1" grid is probably the most important trait that you should look for. When sighting in your rifle at 100 yards, it is easy to adjust elevation and windage because 1 MOA is approximately equal to a 1" square on your target.

This rifle is firing is high and left after the first group.

When you're sighting in a new scope on your rifle, you will first want to get an accurate bore sight in order to get your rounds on paper and reasonably close to your point of aim. Performing a boresight on a bolt action rifle is generally quite easy. All you need to do is to pull the bolt and peer down the bore while your rifle is on the bench rest. Next, without moving the rifle, glance through your scope and make a note of where the crosshairs are indicating. Adjust the crosshairs towards the target bullseye and then repeat the procedure, until your scope appears to be aiming fairly close to the center of your bore sight.

Reinstall the bolt, and then check your scope. It should be centered perfectly on your target with the bags or rest completely supporting the rifle without any outside support. You should be able to set up on your shooting position behind the rifle and place your shooting hand in position over the trigger, and then remove it so that you are completely hands off without the rifle changing position at all.

Take three shots to confirm that the rifle is consistent. Your shots may not be near your point of aim, but since the rifle has been bore sighted they should be on the paper and they should all make a nice tight group. A good tight group demonstrates that the rifle is capable of making consistent and, more importantly, repeatable shots. If your rifle is not shooting consistently, or highly variable wind conditions cause your rounds to drift significantly, it will be very difficult to zero your rifle with any level of confidence. If the wind is calm and the rifle steady on a good solid bench rest, but your rounds are scattered all over the target, it may be time to try another type or brand of ammunition. If you have tried different weights and brands of ammunition and your rifle is still unable to get a good group, it may be time to take your long gun in to an expert gunsmith who can diagnose and hopefully repair the problem.

Good grouping in the middle of the target.

Once you have confirmed that you are getting consistent shots, it is time to adjust the scope so that the point of aim gives you the desired point of impact. Note that the point of aim and desired point of impact are not always the same. If for example you are sighting in your rifle on a 100 yard range, but you want to have a 300 yard zero, your desired point of impact will be higher than your point of aim by a few inches depending on the cartridge you are firing. By the same token, if you want to sight in your rifle for zero wind but are shooting in a consistent 10mph 90 degree cross wind, you will want your point of impact to be to the right of your point of aim.

There are two ways to adjust the scope to get the point of aim and point of impact the same place. One way is to know how far each click of adjustment moves your point of aim and calculate the necessary adjustments. For example at 100 yards, if your point of impact is 4" high and 3" left and your scope adjusts in 1/8 MOA clicks, you would then adjust your scope 32 clicks down and 24 clicks right. The other way is to hold the rifle so that your crosshairs are on your point of aim, and then carefully, without disturbing the rifle in the rest, adjust the scope so that the crosshairs are now over the actual impact holes in your target. This seems counter intuitive, because even though you want the impact to be lower the actual reticle is moving up to meet your actual point of impact. But if you look at the indicators on your turret tubes where you make the adjustments to windage and elevation, you will notice that you are actually moving the point of aim down while the reticle appears to be moving up. The advantage of these methods are that, properly done, they do not use very much ammunition; you can usually get on target with only one adjustment.

When deciding what distance to zero your rifle for, consider the trajectory of your particular cartridge when choosing a point blank zero. Ideally, you will want a zero that gives you a Maximum Point Blank Range, or MPBR. An MPBR is the zero range at which you will need minimal holdover to keep your round on target. When you are in a hunting situation, there is not always time to grab the range-finder and calculate the exact distance to your target or to stabilize your rifle enough to use your Mil-Dot scope to measure the target size and estimated range. By computing the trajectory of your cartridge, you can calculate the ideal MPBR. For example, let's say that you are zeroing your 4-16x50mm scope on a Remington 700 chambered in 7mm Remington Ultra Mag. If you have a zero at 350 yards, your maximum variation out to 410 yards is only +/- 6". For medium game hunting, this will put you at "minute of deer" accuracy for any range between 0 and 410 yards. If we are zeroing this particular combination at 100 yards, the point of impact should be exactly 4.8" above the point of aim. Ideally you should confirm your MPBR zero at the actual distance you are calibrating your scope and rifle combination for: in this example we would confirm zero at 350 yards. Find the statistics of the round you are firing and some ballistics tables and play around to find out your MPBR before choosing a distance to zero your rifle.

Images courtesy of Ruger Firearms.

Understanding and Correcting Parallax

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You see the term often used when describing scopes, but what exactly is parallax, and do you need to concern yourself with the parallax settings of a scope? Parallax describes a situation where the focal plane of the object in the scope is offset from the reticle. If you have parallax, you have an optical illusion that must be corrected. Parallax should not be confused with focus. Parallax compensation changes neither the focus of the reticle nor the focus of the image, but simply moves the planes at which these two objects are in focus so that they are coincident (sharing the same plane).

When looking through a high power scope you can identify parallax by adjusting your gaze slightly. If the reticle changes it's position on the target when you shift your gaze, your parallax is not properly compensated for at that range. The reticle appears to move in relation to the target image because the focal planes are not coincident, making it appear to be a 3D image. Properly adjusted, the reticle should appear to be locked in place as if it were painted onto the target so that no matter how your gaze is shifted the reticle position never changes relative to the target.

Consider the image below:



In this illustration, the point where the focal lines cross and form an X is the location of the focal plane for the target image. Note that it is behind the reticle. When the angle at which you are viewing the image through the eyepiece changes, the reticle position relative to the target image will also change.



In this illustration, the focal plane for the target image and the reticle are the same, so no parallax adjustment is necessary. It is possible to have an accurately placed reticle in the first image only if you are looking at through the scope with your line of sight exactly lined up with the reticle and target image. The problem occurs when your line of sight is not exactly lined up, as the point of aim indicated by the reticle is now incorrect. By eliminating parallax and having the target image and reticle on the same plane, you no longer have to have a precise line of sight: no matter what angle you are looking through the scope at the reticle will still accurately indicate the correct point of aim.

Parallax is usually negligible or not present at all in most low-magnification tactical style scopes, as the scope is too short or the range is not long enough. 1x red-dot style scopes generally are parallax free at any range. Even mid-power hunting scopes have very little parallax, and many tactical models such as the UTG 1.5-6x40 illuminated reticle scope do not have parallax compensation, as it is impossible to quickly and accurately determine range in a dynamic tactical situation.

In high-power scopes used over long distances it is necessary to compensate for parallax. High power scopes are usually equipped with a side-mount turret, or adjustment ring located on the objective bell, so that the shooter can move the focal plane of the target and reticle and eliminate parallax. Some of these rings or turrets are marked with various distances, generally ranging from 50 yards to infinity, to indicate the proper setting to eliminate parallax. While helpful as a general starting place, these factory set markings are not always accurate. I find it helpful to manually determine the proper setting at 50 yard increments and scribe or mark those settings on the scope.

Why is it so critical to get a precise parallax compensation setting? The reason is that the amount of parallax increases with magnification, giving you a larger margin of error at higher powers if your parallax is not precisely corrected. On a high-power variable 6-20x magnification scope for example, parallax appears easy to compensate for at the lower 6x magnification setting. Once the zoom is increased to 20x however, it takes a very fine adjustment to completely eliminate parallax.

To measure the actual parallax compensation needed for a given distance and zoom, you will need to head out to a range with known distances and calibrate your parallax adjustment mechanism. Some parallax adjustment systems such as a side-mounted turret have knobs that can be adjusted to "rezero" the mechanism. In order to get consistent parallax compensation, start with the adjustment knob or ring set on the stop past "infinity".

Starting with a scope that has already been zeroed to your rifle, set up the firearm in a stable configuration aimed downrange at your target using sandbags or a machine rest. Your target should be at the maximum distance possible at your shooting range- preferably 1,000 yards, though shorter distances will suffice. With the magnification set to maximum, sight through your scope and slowly shift your gaze while looking for movement of the reticle in relation to the target. Use the parallax compensation turret or objective ring to adjust the focal planes until there is no movement of the reticle when you shift your gaze. Then, using a fine paint marker mark the point on your ring or turret for this range.

You can continue this process at various distances by moving the target closer and repeating the process. If you wish to do so, you can permanently scribe the positions on your ring or turret so that it is easy to consistently return to that point time and again. Remember, when adjusting parallax using a side-mount turret, always start with the turret set against the stop past infinity and then turn it to the appropriate setting. This ensures that there is no slack or backlash that can throw off your adjustment.