Bullet Ballistics - Shooting Terms You Should Know
May 20, 2015
Long-range shooting is one of the biggest trends on today's shooting scene, big enough that it's driving purpose-designed new rifles and riflescope models. Whether your game is hunting, tactical, competitive or simply recreational, there's a movement within it to stretch the distance at which you can consistently and precisely hit a target. And there should be â€” pulling off a tricky shot on a small target a half-mile away is challenging, gratifying and takes skill and practice.
The movement has adopted several common terms and generated new ones. If you're new in the game, the jargon tossed back and forth at the range or during a match can be bewildering.
Here's a look at several common ballistic terms shooters use, along with details on how they apply to long-range shooting. So next time you hear a shooter say, "My dope is off; I had to come up a full minute to compensate," you won't have to worry about whether it's ok to let your teenager hang out with him.
Simply put, this is the path your bullet takes as it travels toward the target. Projectiles speeding through the atmosphere are affected by gravity and air resistance, with gravity pulling the bullets downward and air resistance causing them to continuously slow. This loss of speed, paired with gravity, causes them to drop faster and faster toward the earth, resulting in a parabolic curve in the bullet's path. Simple apps on a phone or computer can, when fed accurate information, calculate trajectory with admirable precision, enabling shooters to compensate and hit their target.
When a bullet exits the muzzle of a firearm, it does so at a certain speed, which is termed velocity (specifically, muzzle velocity). As it encounters air resistance, it immediately begins slowing, and it will continue to slow until it runs out of steam and its curving trajectory takes it to earth. Velocity plays a big part in how much energy a bullet carries to the target, and because every bullet continually sheds velocity, energy diminishes as velocity diminishes. In the hunting world, velocity is important for an additional reason: to reliably cause a bullet to expand, or mushroom. Without expansion bullets tend to poke a knitting-needle size hole, which kills much more slowly and less humanely.
Knowing the specific velocity of your chosen bullet when it exits the muzzle of your particular rifle is also critical to accurately calculating its trajectory. While ammo boxes or manufacturers' websites often offer velocity specs that come close, nothing replaces actually measuring velocity as fired from your rifle, which is easily done with any decent-quality chronograph.
This term, usually abbreviated to "BC," refers to the aerodynamics of your bullet. A long, sleek bullet with a very pointy tip and a boat-tail base slips through the atmosphere much easier than a blunt-nosed, flat-base design; thus it maintains its velocity much more efficiently. Bullets with high BC's are critical to long-distance shooters: They have a flatter trajectory, drift less in the wind and maintain energy and expansion-causing velocity better.
BC numbers are determined by comparing a projectile's aerodynamic characteristics against a standardized model, several of which exist. Most common is the G1 model, but modern shooters are trending toward the G7 model, which most feel more accurately represents extreme-range trajectories.
BC numbers are commonly represented by a decimalized portion of a whole; for instance a G1 BC of 0.370 could represent a common, somewhat blunt bullet; a G1 BC of 0.600 or higher is considered very good indeed in terms of aerodynamics. All of this, of course, also depends on the caliber and density of the bullet, as well as other factors.
Bullet Wind Drift
This ballistic term is pretty self-explanatory: As a bullet travels, any wind present exerts forces that cause it to drift. Most commonly, the drift is lateral, but in rare occasions â€” such as when shooting parallel to a very steep mountainside with a stiff wind traveling up or down it, wind can actually push your bullet vertically.
A crosswind that is perfectly perpendicular to the line of your bullet's travel is typically called a "full value" wind, because it exerts the most possible wind drift. A wind straight in your face or from straight behind you is called "no value" because it has no discernable effect on the path of the bullet.
Learning to judge and compensate for wind takes a lot of application. In some areas, the shooter might have to judge and compensate for several different winds of varying direction and strength between the firing position and the target.
Many, too, are the methods and tricks for evaluating wind â€” but that's a topic for another time.
Drop is closely related to trajectory. As air friction robs a bullet of its velocity and gravity exerts its irresistible pull, a bullet drops toward the earth. Compensating for it is a matter of calculating how much drop you have at a whatever distance your target is, then holding over it â€” pointing the gun high, if you will â€” so that your bullet drops into the target.
In days of yore, hunters did just that â€” aimed whatever distance over the target experience and their gut told them to â€” and hoped for the best. Today, with laser rangefinders to tell us the exact distance to the target; calculators that tell us exactly how high to hold; and scopes with sophisticated holdover reticles or with elevation turrets that enable us to adjust the crosshairs the precise amount needed to put the bullet on target, a good rifleman can hit small targets at impressive distances indeed.
Although some shooters still refer to drop in inches, there are much more effective units of measurement. Which takes us to the next term: Minutes of angle.
Minute of Angle (MOA)
This term refers to a concept that some shooters struggle with. It's an angular rather than linear measurement, and it's particularly useful because it works seamlessly with the inch system common in the U.S.
To start with, one MOA measures 1.047 inch at 100 yards. Let's just call it an inch. At 200 yards it's double that. At 700 yards, it's seven times that. And so forth.
While it's not exactly the same as an inch, it's close enough for practical purposes: At 1,000 yards 10 MOA measures 10.47 inches. That puts it less than one-half inch away from perfect correlation with the inch system, and if you can shoot well enough to tell a half-inch difference at 1,000 yards, you need to quit reading this article and go start winning championships.
Where MOA comes in really useful is in adjusting for hold at long range. For instance, shooters can either use a scope with a reticle that provides a one-MOA grid to hold over and off for drop and wind drift, or they can dial corrections into an MOA-marked target turret that adjusts the internal crosshairs, and then hold right on.
The windage and elevation turrets on most scopes are marked in either MOA (with quarter-MOA positions, or four "clicks" per MOA) or mils (we'll get to that next). Dialing in MOA is much simpler than in inches. Consider this: While each click on your scope equals roughly 0.25 inch at 100 yards, how much does each click equal at 650 yards? Umâ€¦ 1.62 inches, right? And suppose your drop chart calls for 132.5 inches of elevation adjustment for 650 yards. So 132.5 divided by 1.62 isâ€¦ Sheesh.
The great beauty of the MOA system is that, as a tiny slice of the 360-degree sphere in which you stand in the middle, an MOA is an MOA whether at 100 yards or at 650 yards. If the drop chart on your ballistic calculator calls for 19 MOA at 650 yards, dial in 19 MOA and shoot the target.
Like MOA, the milling system is based on an angular measurement rather than a linear measurement. Military shooters claim that it's superior to the MOA system. Perhaps it is superior for mechanical rangefinding, and I'll admit that's an area where I'm weak; I typically use a laser rangefinder to determine distance. Yep, I know: one of these days the battery will go dead and I'll be SOL.
Basically, the mil system is used in much the same fashion as the MOA system, the main differences being that the mils don't play as nice with the inches and yards that most Americans are accustomed to, and elevation and windage adjustment turrets on the scope are marked in mils and tenth-mils.
Shooters often throw this term around casually, as in, "Let me just dope the wind hereâ€¦" but in reality the term is an acronym for "data on previous engagements." Not engagements of the hostile kind, just shots engaging any kind of target. Many, even most, of the best long-range shooters keep a logbook in which they record conditions, shot distances and results every time they practice or compete, and those records can prove a valuable resource for later shoots.
This refers to atmospheric pressure, and we're dancing with advanced ballistics here, but in essence air density â€” and thus the amount of friction it exerts on a projectile â€” is strongly affected by temperature and altitude. And as we learned above, air density has a dramatic effect on velocity, which in turn affects how much bullet drop we have to compensate for.
Experienced shooters can make a pretty effective guess at temperature, and anybody with a topo map can determine altitude. Plugging those basic numbers into your ballistic calculator will get you close, but to get it just right you'll need to use a hand-held weather station such as one of the outstanding units by Kestrel. It measures actual pressure, including the influence of temperature, altitude, weather fronts and so forth. Really advanced Kestrels, such as those with on-board ballistic software, can be programmed with your cartridge, bullet and rifle's data, after which they'll calculate and provide drop and wind holds based on actual, current conditions right at your shooting position.
Getting on Target
In addition to knowing common ballistic terminology, many skilled long-range shooters utilize ballistics calculators to help them hit targets as distances stretch. With these tools, the shooter can plug in various factors such as bullet caliber and weight, muzzle velocity and environmental elements to determine a bullet's trajectory.
To help successfully complete some of those difficult long range shots, check out Federal Ballistics, Federal Premium's new ballistics app for smartphones and tablets.