Riflescopes and the .338 Lapua Magnum – An OC Guide
The use of the .338 Lapua magnum by coalition snipers during the wars in Afghanistan and Iraq changed the battle-scape dynamics as enemy forces were forced to reconsider using their extemporized battle wagons that were fitted with improvised laminated steel armor plates.
This was because this magnum cartridge can easily penetrate such improvised armor, and kill the occupants inside the wagon.
The same also applied to improvised shields used by enemy snipers in the Iraq war.
It is this proven battle-worthiness (quality) that made it one of the most ideal cartridges for use by coalition snipers. As expected, there is need to describe what this cartridge is.
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The .338 Lapua Magnum
This is a bottlenecked, rimless rifle cartridge designed as a centerfire ammunition by Nammo Lapua Oy – in collaboration with Accuracy International (a British rifle maker) and the Finnish ammunition firm, SAKO – in 1989.
It went into mass production in the same year, and was officially adopted by the Nordic Ammunition Group in 1998.
As a centerfire cartridge, its primer is a separate, replaceable component located in the center (middle/core) of the case head.
This primer location makes the cartridge stable as it can withstand rough handling, while the cartridge base can withstand high pressure which ensures that the cartridge does not go off if it falls on a hard floor (as opposed to a rimfire cartridge whose protruding rim can be triggered by such [fall] impact).
The use of a detachable primer also allows for thick metal to be used to form the cartridge case. This thick cartridge case has a strong base, and consequently demands high pressure to be applied on the primer so as to ignite it.
Accordingly, the bullet exits the rifle at a high velocity and with great momentum (which means that it possesses lots of energy).
This explains why the .338 Lapua magnum cartridges can easily penetrate standard (and slightly better versions) military body armor when fired at a range of 1 kilometer (1000 meters) from the target.
This also makes it an ideal long-range, high-powered cartridge to be used in military-grade sniper rifles.
Depending on the barrel length and seating depth of the cartridge in a rifle, as well as the mass of cartridge powder charge; the commercial-variant .338 Lapua bullet that weighs 16.2grams can attain a muzzle velocity that ranges from 880meters-per-second (mps) to 915mps. This gives the bullet an average muzzle energy of 6525joules.
According to third law of Newtonian (motion) physics, every action (such as firing this cartridge) has a reaction (in the form of rifle recoil).
Therefore, the high muzzle energy of this cartridge generates extensive rifle recoil that can misorient an attached scope.
For this reason, one needs to use a powerful riflescope that is designed to handle the muzzle energy of this high-powered cartridge without compromising the visual clarity provided by the scope.
Unlike regular service rifles which can fire standard high-caliber cartridges, sniper-grade cartridges need to be shot from a sniper rifle as this maximizes their destructive power, as well as ensures that it reaches its target.
Still, one can use a regular rifle with a slightly larger-than-normal rifled bore that can accommodate this cartridge.
However, this tasks one with the tedious repetitive lining up the 2 metallic (iron) sights – the rear sight and the front sight – with the target, without the benefit of magnification if the target is far away (but within reachable distance for the bullet).
To eliminate error occasioned with such a repetitive task, one needs to fix a scope onto the body of the rifle; and since this scope comes with a reticle (usually designed as a crosshair, sometimes with a red dot at the middle [core]), one only needs to align the reticle with the magnified image of the target.
Expectedly, it is easier to shoot – and even learn how to shoot – using a riflescope as compared to using the iron sights.
For people with poor vision, or older shooters, the riflescope allows for accurate aiming of targets. This explanation allows for a simple definition of what a rifle scope is.
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What is a Riflescope?
The rifle-scope is simply a telescopic sight that is fixed on a rifle where it serves as the primary optical sighting device.
It uses a refracting telescope equipped with a graphic image pattern called the reticle to acquire a fairly accurate aiming point.
This reticle is mounted inside the scope at an appropriate distance from the eye-piece lens. In some models, optoelectronics is integrated in the telescopic sight so as to create a night-vision riflescope.
Despite being a special-purpose telescopic sight, the riflescope is still classified using normal telescopic sight criteria – and these are the objective lens diameter (in millimeters[mm]) and the optical magnification.
For example, the denotation 20×40 on a riflescope means that optical magnification is 20 times, while the objective lens diameter is 40mm.
Broadly, riflescopes are categorized as fixed magnification or variable magnification, with the variable magnification models having an inbuilt zoom mechanism that allows for adjustment of the magnification power.
Parts of a Riflescope
The riflescope is a composite device made up of different parts – some moving and some static/fixed – which work together in a synchronized fashion to create an optical sighting unit. The standard parts are described below.
This forms the main body of the riflescope chassis, and it connects the objective lens to the eyepiece. The most important measurement for the tube is its width (diameter).
Tube diameter ranges from 0.75inches through 1-inch to large wide-diameter tubes. The tube allows for light transmission, as well as holds some of the optical components in the riflescope.
As expected, wide-diameter tubes can accommodate larger optical components, which allows for better light management and definitely better optics. Also, wide-diameter tubes have a thick, robust case.
This is the bell-shaped part of the scope that is farthest from the eye of the shooter, and it houses the objective lens.
Some feature an adjustable objective ring which changes the position of the objective lens in the bell hence adjusting the range of view of the scope.
This adjustable ring is calibrated so as to show how much the objective lens changes position from its default state.
This is the bell-shaped lens housing that is closest to the viewer, and it houses the ocular lens. It is also called the eyebell.
This is found only in variable-magnification riflescope, and it connects the eyepiece to the tube. It serves to adjust the magnification power of the scope. It is calibrated, and shows the range of magnifications possible.
These are located at the center of the tube, and there are 2 main types:
It is usually covered by a cap, and is located at the side of the tube. It is called the windage turret. It serves to adjust the left-to-right orientation of the bullet impact, that is, it allows one to adjust where the bullet is to hit the target, either to the right, or to the left.
It is located at the top of the tube, and it serves to move the bullet impact up or down.
Some riflescopes come with additional dials, with the 2 common ones being the:
- Side focus dial.
- Illuminated reticle dial which serves to adjust the intensity of reticle illumination.
Operational Design of a Riflescope
When the afore-described riflescope parts are integrated together into a functional scope, the corresponding operational design allows it to achieve an optical magnification and sight accuracy that allows for practical aiming of targets.
Most rifles feature a rating of the maximum distance between the shooter and the target that the bullet can reach.
Expectedly, the scope magnification must be able to effectively magnify a target located at the maximum distance possible – with some scopes allowing greater magnification, and such a riflescope can be used as a military-grade monocular.
When using the riflescope, the eye of the viewer must be positioned at some distance from the eyepiece.
This length is called the eye relief, and it ensures that rifle recoil does not cause the riflescope to hit the eyebrow, and in some cases, the eye itself.
The operational design also determines the field of view of the riflescope. This is the maximum width of the area that can be viewed by the riflescope.
This width is expressed in feet when the viewer is located 100 yards away. Therefore, a field of view of 30 feet means that if the shooter is located 100 yards away from the target area, an area that is 30 feet wide is viewable through the scope at any instance.
Also related to the field of view is the Minute of Angle (abbreviated as MOA in riflescope ratings). The MOA is a unit (standard) of measure equal to 1.0472inches when the shooter is positioned 100 yards from the target.
This MOA is used in the Windage and Elevation Adjustment dials to move the bullet impact along a left-right axis and an up-to-down axis respectively.
For instance, in a riflescope whose dials feature a ¼ MOA per click (rotating the dial produces click sounds as the user cannot see the dial), then the bullet impact is moved 0.25inches for every click if the target is 100 yards from the shooter.
Some modern riflescopes have substituted the MOA with a more sensitive calibration measure, the milliRadian (abbreviated as MRAD).
This eliminates the need for measurements being referenced at 100 yards. A milliradian is defined as one-thousandth of the distance between the shooter and the target.
Therefore, a riflescope with a 0.1MRAD rating allows the shooter to move the bullet impact 10mm per click if (s)he is positioned 100 meters away from the target.
The objective bell of the riflescope allows for light transmission to the eyepiece, and the amount of light transmitted is dependent on an optical element called the exit pupil.
The exit pupil is a mathematical quotient obtained by dividing objective lens diameter (in mm) by magnification power.
Therefore, the exit pupil for the 20×40 riflescope is:
40mm ÷ 20 = 2mm.
This exit pupil determines the amount of light that reaches the eye, which greatly impacts performance if the riflescope is used in poorly-lit environments where low amounts of light due to a small exit pupil would make it difficult for the shooter to see the target.
Therefore, an exit pupil of 4mm and above is recommended if the riflescope is to be used in the evenings, early mornings, or in dimly-lit enclosures.
This also explains why variable-magnification riflescope deliver superior performance as compared to the fixed magnification models, as the exit pupil can be precisely adjusted to fit the lighting conditions in both dimly-lit and brightly-lit environments while the fixed-magnification models have their exit pupils optimized for a specific range of lighting conditions.
There is also need for diopter adjustment. The diopter which is located near the eye-bell is used to align the focus the reticle, not to the target, but (to align) with the eye.
It allows the user to see a perfectly clear reticle (and not fuzzy crosslines).
There are various types of reticles used, with the 3 common ones being:
- Duplex reticle.
- Bullet-drop compensating reticle.
- Ballistic (or mil-dot) reticle.
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What about Parallax?
The parallax error is one of the most misunderstood concepts with many considering it a focus error. This is because it occurs when the reticle and target are at different optical planes, and this results in variance as to where the point of impact should be.
It usually occurs as the target moves further away from the shooter; as initially, during slight head movements, the reticle and target moved together, but when the target is farther away, similar head movements cause the reticle to appear to float off the target.
This error can be corrected using an adjustable objective ring or by dialing the side-focus dial. This is possible as the positioning of the objective lens relative to the reticle is adjusted so that the real image is perfectly aligned on the same optical plane as the reticle.
The alignment of lenses with the reticle is well-optimized in a special category of variable-magnification riflescopes called first focal plane (FFP) riflescopes.
In these FFP models, changes in magnification equally affects the reticle focus and target lenses. This means that the reticle is magnified when a higher-than-default magnification power is used.
How it Works
To begin with, the riflescope must be mounted firmly on the rifle for it to be used. This makes mounting the most critical factor in determining the performance of a high-quality riflescope.
It should also be mentioned that no level of riflescope adjustment can overcome the errors caused by poor mounting.
This also makes over-adjustments a cause of perceived errors. Therefore, proper mounting is a precondition for proper scope usage.
This also explains why a noob shooter can shoot relatively well after simple bore-sighting of a well-mounted riflescope.
Regarding its mode of operation, the riflescope is a specially designed refracting telescope that operates as a dioptric telescopic sight as described hereafter.
Light rays from the object are transmitted through the convex objective lens where the rays are refracted (bent) so that they can converge at a single point called the focal point – where the focused light rays then create a crisp clear, but very small image of the target object.
Basically, a far-away target is converted into a miniature image that is created near the eye of the shooter. This real image is formed just in-front of the reticle, which explains why the reticle is placed quite near the focal point of the objective lens.
Because the normal eye lens lacks the magnification and resolution power to view this miniature real image, then the image needs to be magnified to a size where a normal eye can pick out details from it.
This magnification is done by the ocular lens which allows one to view a magnified image that is marked by the reticle. This explains why one can accurately aim at a particular body region when the target is far away.