Caliber, Bullet, and Cartridge Selection
When starting a new rifle build for someone two key questions I ask are; What is your target and how far away will it be? Just like you wouldn’t use a bazooka to kill a mosquito because it is not the right tool for the job, we try and tailor our caliber, bullet and cartridge selection to the task at hand.
A quick definition of terms:
A bullet is the object being propelled down the barrel. Bullets come in various shapes, sizes and compositions. Selecting the appropriate bullet for the task is critical for successful shooting. When shooting long-range one must consider the aerodynamic properties of the bullet, this is generally expressed in a B.C or ballistic coefficient number. As long-range shooters we are looking for a high B.C bullet. This high B.C indicates the bullet will be less affected by wind as well as decelerate at a lower rate due to aerodynamic drag (it will stay faster longer).
Caliber refers to the diameter of a bullet i.e 22 caliber, .243 (6 mm), 7mm, .30 cal and so on. Inherently different calibers have different ranges of weight for their projectiles, larger calibers generally being heavier.
For the purposes of this article when I use the term “cartridge” I'm referring to a specifically shaped piece of brass or steel. It is the vessel that holds the propellant. Many different cartridges can fire the same caliber. A simple example of this would be the .308 Winchester, 30-06, and the 300 Weatherby. All three cartridges can fire the same projectile, however, they have different sizes/shapes and therefore different capacities. Generally a cartridge with a larger capacity will launch the projectile at a higher velocity. It is important for us to select a cartridge that will propel our projectile at an acceptable velocity to deliver enough energy on target for its intended purpose.
Unfortunately, despite what some manufacturers might like you to believe, there is no “One cartridge to rule them all”. There are always tradeoffs. You can have a massive bullet with a great Ballistic Coefficient (B.C) shot at some hypervelocity, but physics dictates it will kick like a mule. So, if you are planning on shooting a couple hundred rounds in a day at a competition or for fun, this may not be the direction to go. However, if you are trying to shoot a mountain goat, which is a tough animal, at long range, then this magnum cartridge may be just the ticket. The nature of our target dictates how much energy we need to deliver to it. If we are only shooting steel you only need enough energy to show signs of impact, and with some of the new target cams and sensors available this doesn't require much energy. Since not much energy is required we are able to use a lighter, faster, lower recoiling cartridge. In that application we are seeing many competitors using the high B.C 6mm and 6.5mm rounds being launched around 3,000 feet per second (FPS).
Touching on selecting a Bullet/Caliber/Cartridge for hunting, there are many different factors that play into “Terminal Ballistics” a key one being kinetic energy on target. Kinetic energy is defined by 1/2 mass times velocity squared. Essentially, if you can make an object of equal weight move faster you get more kinetic energy. In the firearms world we express this kinetic energy in foot pounds. Many shooters look at muzzle energy to determine the lethality of their round. This is a fallacy and is the reason I ask the second question “How far away will the target be?” Because unless you are planning on shooting game at point-blank ranges, the real question is not how much muzzle energy does this cartridge provide, but rather how much energy on target will it provide at the distances I am planning on shooting to. This is where we begin to look at energy on target. An interesting fact is that given two bullets of similar profiles and the same weight, the bullet of the smaller caliber will have a higher ballistic coefficient. A good illustration of this can be seen when looking at Sierra’s 7mm 175 grain HPBT matchking which has an advertised B.C of .608 versus the .308 175 grain HPBT matchking which has an advertised B.C of .505. This higher ballistic coefficient means the round will retain more velocity during flight and thusly deliver more energy down range. Another number hunters should be concerned with is sectional density (SD). Sectional density is a number derived from bullet weight and diameter. It is generally accepted that higher sectional density leads to better penetration. Once again, a smaller caliber of the same weight has an advantage. You can probably see where I'm going with this. If the smaller caliber is able to penetrate better and retain more velocity (and the resulting kinetic energy) why would an individual not simply select a bullet of appropriate weight, then determine the velocity at which it needs to travel to have enough energy on target, then select their cartridge based off of what meets those criteria? If there is an overlap in bullet weights such as the 175gr example I provided above I prefer to go with the smaller caliber. Of course, there are other considerations such as ammo availability, price per round, quality of brass available, ect. but this generally a good starting point when deciding what to build a rifle around.