Tag Archives: OPTICAL SIGHTS

SKILLS: Riflescopes: Adjustments & Variable Power

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There’s a lot to learn to really understand riflescopes and make the best choice. Here’s another valuable lesson. Keep reading…

riflecope

SOURCE: NRA Staff

Windage and elevation adjustments in riflescopes are made with either internal or external adjustment systems. Here’s what that means.

Internal: Most modern telescopic sights have internal adjustment systems using threaded, cylindrical knobs or screws in the turrets. The adjustment screws move the reticle assembly in the optical axis inside the main tube against spring pressure. The adjustment screws have clearly marked graduations around their circumference and many have a ball-detent system that clicks as the adjustment screws are turned. Each graduation or click represents a change in reticle position that moves the bullet strike at the target. This is expressed in minutes of angle (m.o.a.) and normally has a value of 1/2, 1/4 or 1/8 m.o.a. per click.

External: Many older scopes have an external-adjustment system built into the mounts and rings. Such scopes remain popular today for certain types of target competition. In this type of scope, the reticle remains stationary within the main tube and the point of the bullet strike is adjusted by mounts having micrometer windage and elevation mechanisms that move the entire scope laterally and/or vertically. These mounts often allow the scope to slide fore and aft to reduce recoil. An advantage of external-adjustment scopes is that the user is always sighting through the optical center of the tube.

As internal-adjustment systems became more reliable and more accurate, the popularity of external-adjustment scopes faded. Today, external-adjustment models are still offered, however the use of such scopes is now generally limited to a few specialized disciplines of rifle competition.

It is important to note that some scope-mounting systems designed for internal-adjustment scopes still incorporate the ability to accommodate some coarse external windage adjustment.

This leads us to the discussion of variable power. Variable-power riflescopes have an internal mechanism to change the amount of magnification within design limits. This consists of an additional set of lenses mounted in an internal tube that slides forward and rearward under the control of a cam attached to the magnification ring. The design of the lens system and its position in the tube controls the amount of magnification.

The popularity of variable-power riflescopes rests squarely on their flexibility. Variable magnification enables the shooter to adjsut the power to suit a wide variety of conditions ranging from lower power (with a wide field of view for fast shots at close range), to higher power (for greater precision at long range). Once considered expensive and unreliable, variable-power riflescopes have become the most popular type as their design has matured and prices have dropped. Todya, the single most popular riflescope is the 3-9X-40mm, which has become a kind of “jack of all trades.” Smaller variables such as 2-7X-32mm remain popular for smaller-caliber rifles, while 4.5-12X-50mm and bigger models are favored for long-range shooting. Despite their flexibility, no one variable fits all applications and that is why there are so many different models.

Despite their popularity, variable-power riflescopes may suffer from certain drawbacks:
ONE: The variable magnification system introduces another level of mechanical complexity and another source for optical error, potentially decreasing reliability.

TWO: The movement of the internal components of a variable-power scope can produce changes in zero as the scope power is increased or decreased.

THREE: Variable-power scopes are harder to seal than fixed-power scopes by virtue of the magnification-adjustment ring.
As the magnification increases, the field of view and image brightness decrease, often substantially.

FOUR: Variable-magnification scopes are substantially heavier than fixed-power scopes.

FIVE: Variable-power scopes are more expensive than fixed-power scopes.

SKILLS: Riflescopes: All About Reticles

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This is the second in a series on optic basics, and it covers the most visible component of a scope: the reticle. Read on…

by NRA Staff

basic riflescope reticle

The riflescope’s reticle is the visible reference used as an aiming point to align the gun with the target. There are many reticle patterns ranging from simple to complex. The most popular remains the general-purpose crosshair. However, even the simple crosshair offers choices, such as tapered, ultra thin, duplex, mil-dot, ballistic compensating, range-finding, center dot, center ring and post, just to name a few. Each configuration is intended for a specific type of use and there are multiple versions of all. For example, tapered crosshairs are a popular choice for varmint hunting and duplex crosshairs are a common choice for big-game hunting. There seems to be no limit to the new reticle designs being offered, and most makers offer at least six or more types. Your best bet is to try out several at a local gun store, then consult with experienced shooters or hunters before making a final selection.

Reticles may be illuminated electronically, with tritium or with fiber optics to enhance their contrast against dark backgrounds; this is helpful especially at dusk or dawn or during heavy overcast conditions. Illumination remains an expensive option that may not work well in very cold conditions and has limited usefulness. Still, it has proven a popular addition to many scopes.

reticle choices

The reticle itself may be located inside the scope at the first, or front, focal plane or the second, or rear focal plane. The location is an issue only in variable-power scopes. Reticles located in the first focal plane in a variable-power scope will increase or decrease in size as the magnification is changed while those located in the second focal plane do not change size when the power is adjusted. For this reason, the latter location has become the most popular.

One situation in which a front-focal-plane reticle is clearly advantageous is in scopes with a mil-dot ranging system. This type of reticle employs dots spaced one milliradian apart on the crosshair. (A milliradian is the angle subtended by 3 feet at 1,000 yards.) An object of known size is bracketed between the dots, and a table is used to determine the range based on the number of dots the object measures. With a rear-focal-plane reticle variable, the mil-dot system is only accurate at one power setting. A front-focal-plane location maintains the same relationship to the target throughout the range of magnification, thus enabling mil-dots to be used accurately at any power.

A second benefit of placement in front of the variable-magnification lens system is that the reticle remains unaffected by tolerances or misalignment of the erector tube during power changes. With a rear-focal-plane location, these tolerances may shift point of impact as the power level changes.

In the past, many scope reticles were not constantly centered, meaning they moved off to the side when windage or elevation adjustments were made. Many shooters found this annoying. Today, nearly all riflescopes have constantly centered reticles that do not change position when adjustments are made.

Crosshairs or other reticle patterns are created by laser etching on optical glass or by ultra-thin platinum wires. Some early scope reticles used strands of hair, hence the name “crosshairs.” Others used spider silk…interestingly enough, the silk of the black widow spider, which has a better tensile strength than other types of spider silk.

David Tubb DTR Reticle
David Tubb’s DTR design takes a reticle about as far as it can go, offering built-in aiming dot compensation choices to even account for density altitude.