Do you know your BC from your MOA? Darryl Pace delves into the meaning behind some of the toughest-to-understand jargon in shooting
It’s sometimes easy to forget, if you have been shooting for some time, that we all need a place to start. Throwing around a million hyper-specific shooting terms isn’t always that useful for those people trying to learn their way. There are a whole array of very specific words we use on a regular basis to help tell stories and describe shooting situations, but very few of us take any time to explain them. So I thought it may prove a good idea to pick out a selection of these and explain what they mean. In my experience, even people a bit further down the path of shooting can use technical terms without ever really having thought about their technical meaning, so this may prove a useful refresher for quite a few of us.
Trajectory
Most people will understand roughly what is meant by this, even if they do not fully grasp all the technicalities. Often used as a comparison between cartridges, simply put it’s the path of your bullet as it travels down-range. The actual trajectory is affected by not only the cartridge, and therefore velocity, but also by the weight and dimensions of the bullet – as well as a host of external environmental factors, of which gravity and wind are normally considered the primary concerns.
Ballistics coefficient
This is very much tied to the previous definition, in that the trajectory of a bullet will be partly determined by the ballistic coefficient. This refers to the bullet itself, not the cartridge as a whole. The BC value is in reference to the projectile – in simple terms it’s a measurement of the aerodynamics of the bullet. The higher the BC value the more aerodynamic the bullet, and the less it will be affected by atmospheric conditions. Hence, the flatter the trajectory. Fire two bullets side by side, with the same exacting values in everything apart from the ballistic coefficient, and the higher BC bullet will have a flatter trajectory.
There are two BC standardisations: the G1 model and the G7 model. The G7 model is more accurately representative of modern high-BC bullets. It’s important if you are comparing the BC between bullets that you use the same measurement, otherwise it’s not comparable. G1 and G7 values will be different for the same bullet.
Head
This term courts a little controversy. I recall a number of years back a reader of Sporting Rifle wrote in to criticise the use of the term ‘head’. It had been used in reference to the 55gn projectile of the cartridge. Of course, although it’s a phrase commonly used, it makes no technical sense, even if everyone knows what you mean. The bullet is the projectile of the cartridge, and the ‘head’ of that same cartridge is the bottom of the case. This of course is where the phrase ‘head stamp’ comes from and refers to the data stamped on to the base, or head, of the case. This usually carries the cartridge designation, such as .243 Win, and some reference to the brass manufacturer.
Minute of Angle (MOA)
This is one we see all the time and yet few really know what it means. It’s often said that a rifle will shoot sub-MOA, but how good is that? The reason people struggle to understand Minute Of Angle is it’s not a linear measurement like distance, which we are all used to. It is instead an angular measurement, which works in conjunction with the inches measurement we often see scope elevation and windage designated in.
The measurement of 1 MOA at 100 yards measures 1.047in, which most people approximate to one inch. The difference with an angular measurement is that it will increase with distance. Imagine a one inch line drawn on a piece of paper at 100 yards. Join each side of the line with a piece of string to the same single point 100 yards away and you have a 1 MOA arc. If you keep that fixed angle out to 200 yards the two lines diverge further and the measurement between the two lines will be 2MOA. The line on the paper is now two inches.
One of the reasons this is so useful is the ease when trying to dial in adjustments at range. Most scopes are adjusted in 0.25 MOA increments, which at 100 yards is basically a quarter inch. At 200 yards this will adjust your scope half an inch. If you know your bullet drop is 51.3in at 500 yards, it takes some maths to work back extrapolation to the linear measurement, which is 39 clicks. If on the other hand your drop chart tells you it’s 9.8 MOA, dial in 9.8MOA and you are good to go. To concrete the idea of the angular system, a 1in group at 100 yards is, roughly speaking, a 1MOA group. A 3in group at 300 yards is also a 1MOA group.
Jump
Often used in reference to homeloading, this refers to the distance a bullet has to travel from the neck of the case before the first contact with the lands of the rifling. For many reloaders, optimum loads are quite often found with the bullet seated close the lands of the rifling, minimising bullet jump.
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