Ballistic Buzzwords

by Dan Rogers
GunsmithUSA Contributor

Seemingly every couple of weeks a new bullet, whether complete cartridge or bullet component for reloading, is released. There appears to be a fascination with long range shooting and hunting. This race to be the flattest and fastest is filled with jargon and abbreviations. But what exactly do they all mean?

The most common buzz words that tend to float around out there with these “new” uber flat super bullets are ballistic coefficient and occasionally you see the term “sectional density”. Now, you do not have to have an advanced degree in physics or fluid dynamics to understand these terms and how to apply them to your shooting. They do however take a little thought. I figured I would try my hand at explaining them and hopefully do so without boring everyone with mindless technical minutia.

First off is ballistic coefficient or “BC” as it is commonly abbreviated. BC is in fact a mathematical ratio. This ratio tells us essentially how streamlined a bullet is or in effect how efficiently it will fly through the air. This measure is expressed as a numeric decimal. The higher a bullet’s BC the longer it will take for the air the bullet passes through to slow it down. SO, if you are still reading and have not skipped to the next article already, it would stand to reason that the ideal shape for a high BC bullet is long and slender.

I know sometimes it is difficult to visualize things moving through the air unless you are a pilot or aeronautical engineer or the obligatory “rocket scientist”. Let’s take boats for example—hopefully they are easier to visualize. A bass boat has a long slender pointy hull and moves very quickly and efficiently through the water. On the opposite end of the spectrum tanker ships and barges have very broad wide hulls and bows. The large flat nosed boats have a much higher resistance to moving through the water. Hopefully that helps and if it does not maybe the following illustrations will help. First we have a flat point bullet photographed using a special method that allows us to see the air currents around the bullet in flight. Flat point bullets have very low BCs and the air resistance has plenty of area to create drag and thus slow the bullet’s velocity relatively quickly. Up next is the pointed bullet. It is easy to see that the pointed bullet has less area at the front of the bullet along with a more streamlined profile. So it stands to reason that the pointed streamlined bullet would be less affected by air resistance and go a longer distance before dramatically slowing down.

 

 

Let’s take our comparison a little further and throw some hypothetical numbers in here. Let’s say just for argument’s sake these two bullets are both 150 grain 30 caliber bullets. Our flat bullet’s BC would be something like .185, and our wonderful streamlined pointy bullet would have a BC of .408. One would fully expect the pointed bullet with the .408 BC, nearly four times the BC of the flat bullet, to shoot much flatter because the projectile would not decelerate as quickly as the flat nosed bullet. I realize that nearly no one uses a flat point bullet anymore in a high powered rifle. I was merely using the stark difference to illustrate the basic principle.

Now for sectional density. If you haven’t already fallen asleep or chucked your computer out the window, sectional density is a ratio of a bullet’s weight to its diameter. Sectional density then is expressed as a numeric decimal, and to grossly oversimplify, it is how long and thin a bullet is versus how short and fat it is. Allow me to illustrate. In the photo below we have two bullets they both weigh 147 grains. The tall skinny one on the left is a 6.5mm (.264 caliber) and the short fat one is a .308 caliber. For numbers sake the 6.5 has a sectional density of .301 and the .308 has a sectional density of .221. Two things should immediately stand out, one is that the long skinny bullet has a much more aerodynamic shape; and two the long skinny bullet should penetrate a target much easier.

 

So whether you are a long range shooter or hunter high sectional density high BC bullets offer many advantages. The longer streamlined bullets offer flatter trajectories and are less affected by cross winds. This is critical whether you’re punching paper or trying to drop that trophy buck on the mountain across from you.

I know you’re thinking how does this apply to me, and how does it apply to my shooting? Allow me to explain how it has changed my shooting. The more I have become a student of how a bullet’s shape affects its flight and how it reacts as it strikes a target it has changed how I select my cartridges and components. I started my serious shooting and hunting with a 30-06, and quickly figured the faster 300 magnum had to be my ultimate flat shooting hard hitting laser beam round. It most certainly was but at the expense of a substantial increase in recoil. I loved the trajectory advantage the 300 offered me over the 30-06. My correction at distance on game left me a lot more room for error, and I did not have a substantial correction until I reached a considerable distance. I did not like the recoil because it was hard to keep game in the scope when the trigger broke to spot my hit.

All of that changed as I started running the numbers and pouring over the ballistics tables. This was about the time the 6.5 Creedmoor hit the market back in 2009. I found the Creedmoor offered me the same trajectory as my 300 mag. I snagged a rifle and never looked back. I have hunted and shot it with great success these past eight years. The cartridge offers the trajectory of the 300 with the stopping power of the 30-06 and the recoil of the .243. All of my hunting buddies at the skinning shed wanted to know what new 300 mag bullet I was shooting. I would say it is plenty sufficient.

I didn’t set out to focus on a cartridge, but the mid weight 6.5mm bullets (120-147 gr) brilliantly illustrate how BC and sectional density work. They also prove less can be more when the proper balance is struck. I will admit on a personal note I am happy to see the Creedmoor succeeding. It has quickly proven itself everywhere from shooting matches to the African plains. I truly believe it is the 30-06 of the 21^st century. I know I have royally stepped in it by taking a strong stance on a cartridge feel free to shred me for it in the comments. I waded into dangerous waters, but I was simply trying to prove a point on how to simply and practically apply these ballistic buzzwords.