By: Instructor Michael Lake
So you need a scope for your precision rifle and there are just too many confusing options... let me take a few minutes to help you sort some of this out. There are a lot of other articles written by a lot of other people covering some of this stuff, albeit partially, so here's my effort to put some of the information in one place:
Question 1: What do all the numbers mean?
Answer: The first number or set of numbers refers to optical power, which is measured in units of magnification that, in industry jargon, are referred to as "X." A 4X scope magnifies the image 4 times. In other words, a 10 inch target viewed through a 4X optic would appear the same size as a 40 inch target at the same distance viewed with the naked eye. Fixed power scopes (scopes that do not allow the level of magnification to be adjusted) give one number for the optical power. Variable power scopes allow the user to adjust the magnification and "zoom in" (or out), and give 2 numbers separated by a dash. These numbers represent the magnification range of the scope, for example: a 4-16X optic gives the shooter a magnification of 4X at its lowest setting, and up to 16X on its highest setting.
The next number is the diameter of the objective lens, the target end of the scope. This is typically measured in millimeters. Thus, a 3-9X40 scope has a 40mm diameter objective lens and allows the shooter to adjust the magnification between 3 and 9 power. Objective lens size is important because it is a determining factor in the "light gathering" ability of the scope. The larger the objective lens, the more light can get in, meaning the image appears brighter. This is particularly important in scopes with a lot of magnification as the more the image is magnified, the darker it appears.
HINT: When shooting in low or failing light, cranking down the magnification may brighten up the view through the scope.
Question 2: How much glass do I need?
Answer: If this is a "precision" gun, the implication is that you will be shooting stuff that is either small or far away (or both) and the amount of magnification you need is a function of these two figures. More isn't always better. More magnification means less field of view, less light gathering, and less clarity. So magnification needs to be balanced with the intended use.
The maximum useful range of a center-fire rifle round is based on a few velocity thresholds. For the sake of simplicity, let's say that the maximum accurate range is generally regarded as the distance where the projectile's velocity falls below the speed of sound, or less than 1,126 feet per second at sea-level. When this happens, the bullet flight path can be unpredictably deflected, or (depending on the type of bullet), it can even flip over and start flying tail-first! So our first figure is based on this distance.
The second figure we want is target size. According to the International Council of Ophthalmology, The maximum angular resolution of the human eye is approximately 1 arc minute, (one minute of angle), which is essentially one inch at a distance of 100 yards. However, just because you can barely see it, doesn't mean you can identify it well enough to fire a rifle at it. The 20/20 line on a Snellen Vision Chart uses block letters that are 1/3" tall at a distance of 20 feet - that is 5 arc minutes, or 5 MOA. And The United States Sign Council (an organization that develops standards for traffic safety signage in the US), uses a standard "visual index" of 30, meaning 1 inch of letter height per 30 feet of distance from the viewer. So in order to be very legible with the naked eye from a distance of 100 yards (300 feet), signage lettering would need to be a minimum of 10" tall. These are important numbers to shooters to be able to differentiate between a coyote and someone's dog, or to differentiate between a target and a hostage, for example. For a humane, ethical kill on most medium to large North American game animals, the vitals area is 8 inch to 18 inches in diameter. This is large enough, and on large enough animals, that a magnification of 1X to 2X per hundred yards of anticipated distance is adequate. For the Police Marksman who needs to instantly end a hostage situation with a precise shot to the roughly 2 inch by 4 inch cranial ocular cavity of a violent criminal, 2X to 5X of magnification per hundred yards would be reasonable, though more might not hurt, even though statistics favor the precision rifle shot in law enforcement being made within 100 yards. For varmint hunters shooting at much smaller critters, 3X to 8X magnification per hundred yards of anticipated shooting distance isn't unheard-of.
So let's put those together for some common calibers:
.223 Remington / 5.56 NATO - Minus it's use in military or law-enforcement, this caliber's scoped-rifle applications are typically varmint hunting or target shooting. With a few exotic exceptions, this caliber goes transonic well below 1000 yards, typically more like 700 to 800 yards, with most practical precision shots on larger game taking place within 300 yards due to a lack of a expansion or fragmentation effect beyond that range. Applying the above formula, depending on the type of shooting, one might consider an optic with a max power in the 8x to 16x range for the majority of uses. For varmint hunting at longer ranges, up to 32X might be considered, which can make a woodchuck look like a grizzly bear.
.22-250/.204 Ruger/.220 Swift, etc... High-speed varmint rounds in these, and similar calibers, buy the shooter more explosive impact on varmints and pest animals with extreme velocities up to or exceeding 4000 feet per second. Lightweight bullet use, however, means wind resistance scrubs off all that velocity pretty quickly, with most of these rounds barely getting to 1000 yards before going transonic. My suggestion for scoping one of these is to consider where you will be shooting and what distances you expect to be shooting at. If you know that the majority of your shots will be at burrowing vermin out to 400 yards, a maximum magnification between 12x and 32X may be in order, usually leaning more towards the higher end.
.308 Win/7.62 NATO - This is probably the most popular do-it-all cartridge in North America, used for everything from varmint hunting to medium-large game and everything in-between. The venerable .30-06 would fall into the same general ballistic category. Rounds can be kept supersonic in these cartridges to 1200 yards or a little bit more, though maximum effective range based on its ability to ethically take game animals is usually limited to 800 yards or less. For a dedicated long-range rifle in this caliber, you might consider up to 25X, though for general use, a max of 16X to 20X should handle everything this cartridge is capable of.
.300 Win Mag/.338 Lapua Magnum - These rounds have a whole lot of power and their use on all but the largest North American game is probably overkill and lots of meat damage without careful bullet selection. The benefit of these cartridges is the increased accurate range that can be gained with heavier, longer bullets at faster velocities. These calibers can keep bullets supersonic in excess of 3000 yards. That said, making such a long-range shot on a game animal may be less than ethical, as all but the best marksmen would be more likely to wound an animal at those distances rather than harvest it humanely. Optics in the max range of 20X to 25X are most suitable on rifles of this caliber.
Question 3: What is a reticle and what type should I get?
Answer: The reticle is a fancy name for the view you see through the scope, specifically the aiming reference such as crosshairs, mil dots, hash marks, etc... For years, it seemed variations on the plain-Jane duplex reticle were about all you could find. In the past decade, with the increased interest in "tactical" style shooting, there has been an explosion of new reticle types on the market. I personally think this is a good thing, but like most good things can be taken too far. The more stuff you put in the reticle, the more things you have to think about. If the reticle is too busy, it can be slow and confusing to use.
Standard Duplex Mil Dot Mil Hash
My personal preference is a mil-hash reticle such as Leupold's TMR or Vortex's MRAD, which leads us to...
Question 4: What are mils and MOA?
Answer: "Mil" is short for "Milliradian," which is a unit of angular measure. It's explained like this:
The distance between the shooter and the target is represented
by the radius of a circle with the shooter at the center and the target at the circumference. If a line the length of the radius (represented in the diagram by distance "X"), is laid along the circumference of the circle, the angle it creates is "1 Radian" which is equal to just over 57 degrees.
The prefix "milli" refers to a thousandth, therefore 1 milliradian is 1/1000 of this arc. So a 1 mil wide target, 1000 meters away, is 1 meter wide. A 1 mil wide target, 1000 yards away, is 1 yard wide. So "mil" optics don't care if you're working in yards, meters, or cubits for that matter. 1 mil is always 1/1000 of the distance between the shooter and the target laid out along the circumference of a circle with that radius.
With this information and some knowledge about the dimensions of the target, you can even use the reticle to estimate the distance to the target.
MOA, or "Minutes of Angle" are based on the degrees and minutes system. There are 360 degrees in a circle, and a "minute of angle" is 1/60 of a degree. There is a slight difference between the mathematician's MOA, and the shooter's MOA. Shooters MOA typically refers to a measurement of 1 inch of target width per 100 yards of distance to the target. So 1 MOA represents 1 inch of target if the target is 100 yards away. 1 MOA represents 2 inches on a target 200 yards away, and so on...
Both are usable systems, it depends on personal preference, but in my experience people seem to find mil's easier to use.
HINT: it is a good idea to match the adjustment knobs to the reticle subtension. If you have an MOA reticle, adjustment knobs in MOA (or fractions of MOA) are beneficial. If you have a mil reticle of some kind, getting 1/10 mil knobs (if possible) simplifies making adjustments.
Question 5: What is focal plane and why do I care?
First Focal Plane and Second Focal Plane (FFP & SFP) describe the placement of the reticle in variable power optics. In first focal plane optics, adjusting the magnification of the scope also adjusts the appearance of the reticle to maintain the correct proportions between the reticle and the target. In other words, the distance between mil dots represents 1 mil of target width regardless if the optic is on minimum or maximum magnification.
FIRST FOCAL PLANE
More Magnification Less magnification
SECOND FOCAL PLANE
More magnification Less magnification
Adjusting the magnification on a second focal plane optic, on the other hand, only adjusts the appearance of the target, not the reticle itself. So while the distance between mil dots represents 1 mil at a certain magnification (usually the highest setting), at lower magnifications it will subtend something other than a mil. This can make consistent ranging or use of the reticle to make holdover shots more difficult.
HINT: First Focal Plane optics tend to be a bit more expensive, but if you intend to use the reticle for ranging or holdovers, it's the way to go.
Hopefully, you find these explanations helpful in selecting an optic with the right features.