What Do the Numbers on a Rifle Scope Mean?
When choosing a rifle scope, you will inevitably have to deal with a seemingly cryptic set of numbers and letters that define its abilities. While not a measure of quality, they do give an accurate grasp of the performance you can expect. Most rifle scopes use a standardized format to give this information, with occasional additions. By understanding these numbers, you can better match your choice to the shooting situations you expect to encounter.
Numbering Format
The numbers on a scope will usually follow a predictable pattern, similar to other optics such as binoculars and telescopes. In the most basic form it will define the magnification power, followed by the size of the objective lens, with the letter 'x' dividing the two. As an example, a 4x30 rifle scope will have a magnification of 4 times, or 4 power. The objective lens size is listed in millimeters, in this case 30mm. Additional information may follow these numbers and such numbers usually refer to additional features included by the manufacturer and do not always follow a common practice.
Magnification
The magnification of a rifle scope tells you how powerful it is, with a larger number indicating more power. A 2x magnification would double the size of what you are viewing, where 4x would quadruple it. Many are capable of variable power, which can easily be adjusted to zoom in or out as needed. Variable power would be indicated by a pair of numbers before the x, telling you the lowest and highest power available. A common variant is a 3-9x40, which can be adjusted anywhere from 3x to 9x. Target scopes are much more powerful, with magnifications anywhere from 12x up to 40x being common.
Objective Size.
The objective lens is the lens on the front of the rifle scope, and its size also impacts the performance by controlling the amount of light entering the rifle scope. As a general rule, the larger the objective lens, the greater the amount of light allowed to enter. This is particularly important at dusk and dawn, as well as during cloudy weather. More light gives you a brighter view and allows the rifle scope to be used in darker conditions. The drawbacks to a larger objective lens is increased weight and added difficulty in scope mounting. Common sizes range from 40mm up to 50mm, with less common models available both smaller and larger.
Tube Size
Generally, most modern rifle scopes are assumed to have a 1-inch tube when determining what size mounting rings are needed. This is not universal by any means. Older rifle scopes can be found with a 7/8-inch tube, and many newer ones are now being sold with 30mm tubes. This is usually not listed in the model number, but it is still a critical number when shopping for a rifle scope.
Other possible lettering
On some models of rifle scopes there may be additional lettering after the objective lens size. One common marking is AO, for adjustable objective. This allows for fine tuning of the lens parallax, which reduces the chance of error at longer ranges. Rifle scopes made for the tactical market may also be marked with BDC, for Bullet Drop Compensator. This is a mechanism that adjusts the cross-hairs according to the range to the target, eliminating the need to estimate the aiming height. A BDC rifle scope will be made for use with a specific caliber, and will not be accurate if used with a different one.
Some manufacturers add markings to identify the type of cross-hair reticle, or any lens coatings. This type of option is unique for each manufacturer.
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