Intro To Long-Range Rifle Scopes
Intro To Long-Range Rifle Scopes
Whether you’re interested in long-range hunting, competitive shooting or hitting steel targets at a mile, the most important piece of equipment in long-range shooting is a targeting scope. For someone new to the sport, choosing a scope can be daunting. Compared to a traditional rifle scope, the cost of a long-range shooting scope is high and it can be difficult to know how to choose. To help make that process easier, here is an explanation of the specifications used to describe long-range scopes as well as some related concepts that you’ll need to understand before making a decision.
Scope Specification Numbers – Scopes are usually described using a standard number convention which looks like this:
15 x 46mm or 5-20 x 50mm
The number or numbers in front of the “X” indicate the magnification level. A single number indicates a single magnification scope and a number range indicates a variable magnification scope. The magnification level indicates how many times larger the image will appear through the scope as compared to your naked eye. The number after the “X” indicates the size of the front or objective lens. A large objective lens is able to gather more light, which improves image quality, particularly in low light conditions.
Mil vs MOA - Mils and MOA (pronounced “M-O-A”) are two different units of measure for angles around which a targeting scope can be built, much like a tape measure could be built using meters or yards. These measurements are used to calculate ballistic solutions, estimate distance to targets, and describe the precision of a rifle. Mil refers to milliradians which are derived by dividing an angle called a radian into 1000 parts. Because the metric system is also based on dividing units of measure into thousands (1000mm = 1m), metric distances integrate easily with milliradians. MOA refers to Minute of Angle and relates best to yards and inches. Both work equally well and while Mils is now the primary system used by the US military and most top-level competitive shooters, choice of Mils or MOA ultimately comes down to preference. (For more details on Mils and MOA, see the illustrations at the end of the paper.)
Reticle – The reticle is the part of the scope visible through the eyepiece that you line up on your target, the most basic form being a simple cross hair. Targeting scopes used in long-range shooting are made with targeting reticles which have measurement marks on them. These measurement marks, either dots or lines, are based on either Mils or MOA. There are many specialty reticles but all are based on these same Mil or MOA measurements. One common type of specialty reticle is a “hold off” or “Christmas Tree” reticle, pioneered by Horus. In these reticles, measurements are shown in the form of a grid pattern, as well as on the vertical and horizontal lines. Each of these points represents a possible point of aim, allowing for adjustments to shot placement without having to make adjustments to the turret.
MOA/Mil MOA/Mil HORUS
Internal adjustment range – The turrets or nobs on the top and side of a targeting scope adjust the position of the reticle in relation to the target. This allows for ballistics solutions of elevation and windage to be dialed in. Scopes have a limited amount of available adjustment range. This can be problematic if your solution for a 1000 yard shot calls for 35 MOA of vertical adjustment and your scope stops at 30. In addition to purchasing a scope with a larger adjustment range, or aiming above the center of the cross hairs by using the measurement marks on the reticle, angled scope rails move more of your scopes adjustment range above your zero. For example if you have 60 MOA of total adjustment, 30 above your zero point and 30 below, an angled, 20 MOA rail can shift your adjustment range to 50 MOA above and 10 below, allowing you to use more of your scopes adjustment for longer range shots. As a point of reference, a 1000 yard shot with a typical .308 will require just over 45 MOA or 13 Mils of adjustment.
Focal Plane – Focal plane , either first or second, indicates where inside the scope the reticle is located and as a consequence, how the size of its measurement marks relate to the size of the target at different magnification levels. In first-focal plane scopes the reticle will grow and shrink in sync with magnification levels so reticle spacing will be accurate at all magnification levels. This is useful in dynamic shooting situations at variable and unknown distances. In second-focal plane scopes, the reticle size is fixed and measurement marks are only accurate at one magnification level (typically max magnification) which is more suitable for fixed distance shooting environments.
1st Focal Plane 2nd Focal Plane
Click value – To allow for precise ballistic solutions to be input, each “click” made as the turret is turned adjusts the alignment of the reticle by less than one full Mil or MOA. Mil based turrets typically adjust in .1 Mil increments and MOA turrets move in .25 MOA increments.
Exit pupil diameter - This number varies with magnification levels and indicates the diameter of the image projected out the back of the scope. This will determine how bright the image will appear to your eye. In bright light conditions the human pupil has a diameter of about 2-3mm. In low light, the pupil opens up to about 7mm. If the scope’s exit pupil size is smaller than your pupil in the given light conditions, the image will appear darker than it would to your naked eye. Exit pupil diameter is calculated using the following equation.
Objective Lens Diameter / Magnification Level = Exit Pupil Diameter
Therefore, for a 5-25 x 56 scope at 5x magnification, the exit pupil diameter will be 11.2mm. At 25X magnification, the exit pupil will be 2.24mm. In low light conditions, an exit pupil of 2.24 will be smaller than your eye’s pupil at 7mm and so the image will appear dark. At some point, all long-range scopes will reach a magnification level which will cause exit pupil diameters to appear dim in low light conditions. What’s important to remember is that while a scope may have very high magnification, if the objective lens isn’t large enough, it may be difficult to use those high magnification levels under all lighting conditions.
Field of view – Field of view values indicate the diameter of the image visible through the scope at a given distance, typically given at 100 yards or meters. In other words, if the field of view is 6 feet at 100 yards, then at 100 yards you would just be able to see the top and bottom of a 6 foot tall object through the scope. This image diameter will vary with magnification so a high and low value will be given for variable magnification scopes.
Eye relief – Eye relief is the distance behind the rear lens your eye may be positioned while still being able to see clearly through the scope. To avoid being hit by the scope, eye relief should be at least 3 inches and more is better for higher powered rifles with heavy recoil.
Zero Stops – To help shooters return to their 100 yard zero without having to remember how many clicks up they just dialed on their last shot, some scopes have zero stops, or a similar feature known by a similar name. The zero stop is set when the rifle is zeroed in at 100 yards and provides a fixed reference point when changing firing solutions.
Illuminated Reticles – Illuminated reticles change the color of the reticle against the background from black to some other illuminated color, typically green or red. This can help make the reticle easier to see in low light conditions, particularly when the reticle of a first-focal plane scope appears smaller at low magnification.
Objective outer diameter - This is the outer diameter around the front lens of the scope. This measurement will be larger than the objective lens as it includes the thickness of the scope body. This measurement is important because it will dictate how tall your scope mounts will need to be to provide sufficient clearance between the scope and the barrel.
Parallax adjustment – Parallax refers to the apparent movement of the reticle in relationship to the target if the shooters eye is not looking directly down the center of the scope. Parallax is caused when a change in magnification moves the focal point of the lens in front of or behind the reticle. Parallax adjustment knobs move the focal point back in line with the reticle and eliminate the parallax movement. These knobs are typically found opposite the windage turret or closer to the eye piece. The parallax specification indicates the target range the system is able to adjust for.
Eyepiece outer diameter – This is similar to Objective Outer Diameter except it refers to the eyepiece of the scope.
Tube Diameter – This is the size of the scope in the center where its diameter is constant. Larger tubes typically allow for more internal adjustment.
Mounting length – This is the distance at the center of the scope (interrupted by the turret knobs) where the scope tube has a constant diameter. This is the space where the scope mounting rings will need to fit.
For an extensive review of scopes currently on the market go to; http://precisionrifleblog.com/2014/07/01/tactical-scopes-field-test-overview-rifle-scope-line-up/