Ballistic Buzzwords

Dan Rogersby Dan Rogers
Guns and Gunsmiths 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 21st 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.


23 Responses to Ballistic Buzzwords

  1. Right on Dan, thanks for another article!

    I have never reloaded and never learned how to so I am planning on purchasing Fred Zeglin’s Reloading A-Z course as soon as the American (Obama)law allows AGI to deal internationally (sell to Canada).

    Because of my total lack of reloading knowledge I am confused and have a question for you Dan: Regarding the sectional density, to ME it would seem like the .308 bullet would have a larger/bigger/heavier sectional density than the 6.5 bullet? Am I right or wrong? The reason I ask is I figured that the higher the number for sectional density would mean a heavier bullet and therefor a larger sectional density meaning the .308 would have a larger sectional density “number” than the 6.5?

    Thanks sharing! Cheers.

  2. Good article. Now try to wrap your mind around a 16 inch projectile fired by our now mothballed battleships. It’s ojive looks nothing like the latest “gee whiz” 6.5, but it leaves the muzzle at ~ 2700 fps and hits a target in excess of 16 miles away because it has a BC of 15. On second thought – better not confuse the issue.

  3. Thanks for the question Dana and by no means am I an expert on this topic, but let’s wade off into some minutia! I was trying to boil down things into rather simple terms that could be visualized.

    Now ME is the muzzle energy of a particular projectile. We know ME is related to a bullet’s weight and speed. Where I believe a 6.5 wins over a .308 is that at great distance (I think it starts to gain on it around the 400yd mark) the 6.5’s high BC means it slows due to air resistance less quickly than the .308.
    SO at certain distances the 6.5 with a say 140gr bullet can carry more energy to the target than say a 175 gr bullet at .308 muzzle velocities. This can be shown if you google a comparison of the trajectories to 1,000 yds of both calibers. The 140-142 class 6.5 bullets have higher sectional densities because they are longer and skinnier than the 168-175gr .308 projectiles.

    Sectional density is derived from dividing a bullet’s weight (in pounds) by it’s diameter in inches. SO as a general rule the higher the sectional density the heavier a bullet is in relation to it’s diameter. That is why I chose two bullets of equal weight to photo in my photo regarding SD. They were equal in weight not diameter. The weight of the smaller diameter 6.5 gave it a higher SD than the same weight of the larger diameter .308.

    So you are correct the heavier the weight generally the higher the SD but that has to be taken into account with the bullet’s diameter as well. And yes there are 30 cals with higher sectional densities than the 140 class 6.5 bullets, BUT you have to get above the 190 grain mark in the 30 cals to get that kind of long bullet for the larger 30 caliber. The 200gr SMK comes to mind with something like a .300ish SD compared to the 142gr 6.5’s .290something. At this point in the 30 calibers we struggle to push such a heavy pill in anything besides a magnum at any appreciable velocity. (that’s my opinion no offense to the guys pushing them and the heavier bullets out of there .308’s for specific purposes).

    I hope I didn’t get to long and drawn out on that and I hope it at least sheds a little light on your question. I hope you can get to loading soon.

    • 6.5 sold me when you look at the BC. i had to have a 260 Remington in the ar 10 rifle. still i am going to get one more of those. i love3 the Grendel. not being that knowledgeable has the 30 06 been necked down to 6.5. i think i would like to do that to one of my guns. that seems like it should be a standard here.

      just like you said the 308 has more energy at the start then the 260 Remington which is a 308 case necked down to a 6.5. at 1000 yards the 260 has more energy then the 308. not sure at what yardage the 6.5 takes over but boy does it slice throw the air. it has been my favorite diameter for 5 years maybe more now. not sure why some calibers are more BC then others but they are. is the 50 bmg one?

  4. Nice point Jimmy I just saw your comment. And yeah good call maybe not confuse things!! Artillery and naval guns have always amazed me.

  5. To Dana: I took Fred’s course on reloading and I thought it was Wonderful. He uses the same teaching technique that AGI is known for and is a very good instructor. Even if you never reload, the information is still valuable.

    To All: I like this short article on two of the more common ‘new’ buzzwords. I liked Dan’s conversational tone in the article. I think that he explained the concepts nicely with the simple examples. I am glad the he added his real-life experiences about recoil to the article as it makes the two concepts come alive with his ‘pain’.

    • Thanks Michael for the words about Fred, Fred’s reloading course and AGI. I am an AGI student/graduate and really value their teachings and info. Cheers to ya.

      PS: Dan is a great writer and teacher, and it is great to have him here in G&G.

  6. Mr. Rodgers,
    Nice, well-written and good article. That said, not all shooters are into turn bolts or full auto rifles to the exclusion of all others. You said, “I realize that nearly no one uses a flat point bullet anymore in a high powered rifle. ” That skips over everyone shooting lever action, tube fed rifles, we have to use flat tipped ammunition. Perhaps it is the programmer/analyst in me that is wondering what you mean by high power. 45-70, 444 Marlin, 30-30 Winchester, .44 Magnum lever guns are not something I would want to be on the wrong end of. Good article, just harassing you 🙂

    • there is friction every where. even the wind has friction. now when you compare the BC of those cartridges you will see they have a lot more friction. less friction from the wind and traveling threw the air the less change in point of contact. also, the less loss of speed so there energy will be higher. now if i had the choice of a 44 or a 30-30 i would choose the 44. if i could choose the distance i would choose the furthest distance i possibly could 1000 plus yards. i do love my 45-70 but it is for close quarters. i do not think i have ever shot my 30-30, it is just to much fun shooting ars. the evil black gun. keep up the good work Dan!!

  7. A couple years ago I was trying to decide what next for my AR. I chose to put together a 6.8 SPC upper. It will drive tacks (or 16D nails) at 200 yards, or steel plates at 400 yards.

    It also dropped a deer where it was standing. Upon field dressing it, I saw that not only did it go through both lungs, it severed the arteries at the top of the heart so that it rolled away when I opened it up.

    • i have a 6.8 but the BC of the 6.5 kicks it’s but. besides who wants to own just one gun. pick up a Grendel you will love it to. i to not believe there is a ar platform i do not like. the 50 Beowulf is so accurate also. it just does not have the BC.

  8. To all I appreciate the positive feedback. I will continue to scratch the ole noggin to come up with topics that are hopefully informative.

    To Allan, touché sir, you are correct many people use rifles with actions that aren’t turn bolts or autos as you pointed out. You also bring into the fray what exactly constitutes a “high powered” cartridge. It’s my understanding that even this is a little debated among most shooters. My implication in high powered pertained to the calibers generally used in high powered competition shooting. I was by no means calling the calibers you listed anemic! I wouldn’t want to be on the receiving end of ANY cartridge!

    I mentioned the bit you quoted because I have some reloading books from the late 60’s and early 70’s with data for flat point and round nosed bullets in cartridges such as the 30-06,308, 270, and the lot. It was not an uncommon practice to load such bullets back then for short range shooting whether because the shooter believed the bullets “punched” through brush better or the bullet had better wounding capabilities than a pointed bullet. That was my frame of reference for the quote you mentioned.

    I’m not trying to be snarky just trying to give you the full picture. I think we will constantly debate what is or isn’t “high powered”. A friend of mine and I debated this incessantly when we both only had one rifle. He had a 25-06 I had a 30-30. I enjoy shooting the calibers you mentioned and if you haven’t tried them already I would advise checking out Hornady’s Leverevolution ammo. I have used it in some of my lever guns. They offer it in every caliber you listed. They use flex tipped boattailed projectiles. My first rifle was a 30-30. I still use it on occasion. I did a story on it a while back.

    Thanks to all

    • very true Dan,
      i have not tried those bullets yet. i feel in love with an ugly black rifle. beauty is only skin deep. so i guess i am all right. great article…

  9. Art,
    Somehow I missed your post about the .260 you have. Awesome choice in any platform but especially the AR. To answer your question on has the 30-06 been necked down to 6.5. Yes by Charles Newton in 1913. It was a wildcat that was eventually adopted as the 6.5-06. It is no longer commercially loaded. I am debating building one. Newton is an interesting man to research.

    The 6.5-284 and .264 win mag are worth looking at as well. The 6.5-284 and .264 win mag are commercially chambered and ammunition (though sometimes hard to come by) is commercially loaded. The 6.5-06 is a strictly custom deal. You would have to form your own brass from either 25-06, 270, or 30-06 brass. More food for thought.

    • I just posted a comment to JLP in one of the other Guns and Gunsmiths articles this week “Practical Enhancements for Your AR-15–Part 1” and in it I mentioned a gentleman I met one day at the shooting range and we fired each other’s guns.

      Anyhow Dan on this same day this same gentleman had a .264 win mag and let me, my friend and another fella (someone new I met that day also) shoot one round each through it since all three of us never shot this caliber before. I was second up to shoot it.

      As I watched the owner of the gun and the guy before me who shoot it, each shot produced a rather large flame ball emanating from the muzzle. Seeing this I was a little reluctant to shoot it and they guy before me noticed this and said to me “it’s OK Dana, the felt recoil is about the same as shooting your Benelli MR1 here but the flame ball is quite warming on the face!”. His comment about the recoil put me at ease.

      Oh man, when I shot it I was so pleasantly excited I wish I could have shot it again. I’ll have to ask the fella this weekend if they were factory loads or if they were hand loads.

      It was a REALLY FUN day at the range, despite the pouring rain and my gun getting soaked in the rain for 2 hours while I was distracted with the whole event that day.

      I will always remember the comradery that day and I hope to see many, many more of those kind of days out shooting.

      Thanks for prompting an Excellent memory of the .264 win mag Dan!

      • Forgot to say I took a cell phone video of all three of us getting the chance shooting the .264 win mag, and its special to me when watching it now and again. Good times!

  10. Not to be critical, but the sectional density of a bullet is the ratio of the bullet’s weight, in pounds, to it’s cross-sectional area, in square inches. If d is the diameter of the bullet, the cross-sectional area is proportional to d-squared (or d * d). Sierra’s website explains it here:

    http://www.sierrabullets.com/resources/x-ring-newsletter/index.cfm/xid/24/Sweet-Bliss/

    Wikipedia has a good article on ballistic coefficients:

    https://en.wikipedia.org/wiki/Ballistic_coefficient

    In general, ballistic coefficients depend on sectional density and shape, and are computed with respect to a _standard projectile_ having a ballistic coefficient of 1.0. There are several such standards in use and, of course, for any given bullet the bc could vary depending on *which* standard projectile you’re using.

    • Very correct Robert. I omitted some of the more technical aspects of how these terms are arrived at to explain them in a way the casual shooter could readily grasp.

      I appreciate you input, and Sierra is an awesome ballistics resource. They jam pack their load data books with all kinds of ballistic goodies and formulas. There are now also countless computer programs for comparing such data. It’s a good time to be interested in these kind of things. It makes it much easier and less intimidating for the novice to get up to speed on how these buzzwords apply to their shooting, caliber, and bullet choices.

    • Hello and good day Bill.

      Thanks for passing on a “hello” to me. You must be in Newfoundland (Nl), and definitely the hunting would be GREAT there, eh!

      AGI is great and I always look forward to watching/learning anything I can from them. Kudos to you for pursuing Gunsmithing through AGI – smart move buddy!!

      Cheers Canadian friend.

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