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Sectional Density?
One Person’s Observation
By: Robert Kaldahl
There are many facets of reloading that bear a direct relationship to the final product of your labor. Each decision made by the reloader had a direct bearing on the quality, usage and efficiency of the final round. This article is being written in an attempt to assist mainly those new to reloading and as a refresher for those that have been involved in reloading for a while.
We as reloaders manufacture our own ammunition for various reasons, it may be to save money, improve accuracy, enhance ballistics, and fill special needs (such as low recoil/velocity) and several other reasons. One that very seldom is mentioned is that of enjoyment in the sport.
Sectional Density is supposedly a factor in the Terminal Ballistics of any given load. SD is a factor that may have some determination into Penetration that is also determined by the resistance of the material hit. SD is also affected by the materials ability to resist penetration that results in energy dissipation and projectile deformation which affects the bullets stability and ability to penetrate a given medium. In our case this medium is the body of a game animal.
With all else being equal this article will deal with Sectional Density (SD). Many writers in the past and even a few this day and age have written charts and beliefs that certain SDs are good for specific categories of animals that we hunt. The general consensus is that the lower the SD the smaller and more fragile the game and the higher the SD the larger more robust the animal. This is not without its merits but leaves many factors omitted in a very complex decision. When a bullet’s SD is given the only thing that it is based on is a mathematical formula.
It is calculated with this formula:
Sectional Density =(Bullet wt. in grains)
7000 x ((bullet dia. In inches) x (bullet dia. In inches))
Let’s take a 30 caliber bullet weighing 165 gr.
165gr / 7000 x ((.308 x .308)) = 165 / 664.048 = .248
For the lay person, SD is a mathematical formula deriving a specific number that is generally used to equate that bullets ability to penetrate through a given medium. This formula is based on mass and the bullets cross-sectional area. But what does this number equate to? For many years I read and listened to others admiring the virtues of a bullet with a high SD. I thought that it had to actually mean something so for a long time I jumped right onto the same bandwagon. That is until one day on a hunt in Alaska for Caribou I was using what I just knew was a great bullet! To me this bullet was great! Big Green had made a beautiful 180 grain Spitzer Boat Tail that shot like a dream in a 300 H&H Magnum that I had picked up on a trade. It shot like greased lightening out to and past 300 yards flat as a road across North Dakota. It had a BC of .540 and an SD of .271, I thought this was just the ticket. Well so much for dreams, my first shot was at the grand distance of less than 50 yards, the bou was angling away so going for the off shoulder I let her rip. That great bullet hit the last rib on the left which shattered and the bullet tore a gash all along the left side of that bou! Well after two more shots it went down for the count. Upon skinning and dressing it out we found the majority of the jacket just under the skin in front of the left shoulder, no core was found anywhere from that shot, the other two shots were also found, again the cores had separated and blown apart. Talk about a mess, the only thing that was usable was from about the middle of the ribcage back, the front shoulders were a mass of bloody pulp. The two jackets were found in the on-side lung and most of the damage on the off side was from bone fragments and little pieces of lead. So much for a great SD of .271. Well I also happened to have 40 rounds of 150 grain Nosler Partitions with me so I thought I would at least try them. ( ** Note** This hunt was with a local Tribe in the Dillingham area so there was no limit as long as the game went into the Tribe’s Winter Food Cashe.). So now I go from a 180gr with an SD of .271 to a 150gr with an SD of .226, everything I had read told me that this bullet would be a failure on anything but very light skinned and light weight game. Needless to say, the 150gr Nosler Partitions dropped those bou like the hammer of Thor, not one bullet was recovered out of 8 that I shot, only one needed a second and that was my last one. All of them were shot in a migration and were all within 50 to 75 yards, these animals are not the most intelligent when in a migration, they may get a bit on the fast side but they don’t change their route by much.
When I returned home from this excursion I sat down and pondered what had happened. I came to the realization that SD and this formula does not take into account anything other than basic figures relating to a bullet, only its weight and diameter. This is a very general figure that really has no correlation into the bullets performance. It does not take into account the shape of the bullet, the materials the bullet is made from, the construction of the bullet nor the velocity that this bullet will traveling on contacting your intended target. Thus any bullet with the same weight and diameter will have the same Sectional Density.
This is a very flawed look at something that determines how effective your chosen bullet may perform. Using only a published SD to choose your bullet is like choosing your auto based only on the size of the engine.
Basing SD on weight/diameter neglects other considerations, jacket thickness, core density/alloy, mechanical/chemical bonding of the core & jacket, inclusion or absence of a mechanical partition, point configuration, inclusion of expansion enhancing tips or weakened areas of the jacket and or even the lack of a core such as in a homogonous bullet like the Barnes Triple Shock. There are even combinations such as the Combined Technologies Fail Safe that uses a homogonous forward section with a hollow point to assist expansion and a lead rear core with a steel back cup to keep the bullet from riveting.
One note that should also be remembered is that even a cast bullet in our example weight and caliber has the same SD as any of the manufactured bullets, no matter what the alloy used. Pure lead, 20:1, 30:1, Wheel Weights or even straight Linotype. What would happen if you were to launch a 180gr. pure lead bullet with a semi spitzer profile from your .300 Wiz Bang Magnum at 3300 fps at a 300plus pound Mule Deer at 50 yards. First off you would be lucky to hit it and second your bullet would miserably fail on contact with your target. So what good is SD in this case? Your bullet has an SD of .248 and according to general knowledge it falls in the area that is recommended for this size animal.
Now lets take this same weight bullet, change the nose profile to a metplat 80% of diameter flat point made out of a mixture of wheel weights and linotype. We then launch this bullet at a muzzle velocity of around 2100 fps from our .308 XYZ in a pistol configuration. We have a totally different result, more than likely venison in the freezer and a successful conclusion.
In the two examples above both bullets have the same SD, but one fails and the other provides complete penetration and the required wound cavity to be successful in taking our quarry. Note also that this is using cast bullets not anything fancy. Change the bullets to a 125 gr Nosler BT and our Magnum again fails due to performance of the bullet where our hunter with his pistol is again successful.
These are a couple of very exaggerated cases that make the point very clear, but how about the new reloader, what is available to them in determining what bullet would be best for their application. I don’t profess that I have all of the answers but hopefully this layman’s article will possibly assist at least one person.
Most hunters will go after some species of deer for most if not all of their “Big Game Hunting” so lets for ease and convenience use the common whitetail.
Whitetail deer from my best knowledge fall into the vicinity of 22 sub-species. The smallest is the Key Deer in the Florida Keys ( they may hit the scales at 25 – 30 pounds live weight for a huge deer) to the largest in the northern areas of the continental US and Canada that hit the scales at times well over 300 pounds live weight with some going over 375 pounds. As Key Deer are protected we won’t have to worry about them but just a bit further north in Florida whitetail abound. These too are small and it doesn’t take much to put them down but for all practical purposes you wouldn’t want to use the same bullet for them as you would going after a Michigan whitetail in the Upper Peninsula.
Lets use a .308 Winchester for our hunting firearm and only use it in a rifle. On our hunt in Florida what would we want to consider important for use in the selection of our projectile.
1) Size & Weight of our deer, probably on the order of a maximum of 160 pounds live weight.
2) Distance that the deer will shot at, lets use 250 yards as a maximum.
3) Incidental game that may be taken during the hunt. In this case there may be a good possibility that a good sized hog may appear.
Now lets see what is available in our local Gun Shop in the Reloading Section.
We have a huge selection of .308 bullets, they range from 100gr. to 220gr. The bullets available at this shop come from 6 different manufacturers and your reloading mentor suggested that you look at the 150 – 180gr weights. As you look over the boxes of the recommended weights you see that there are 11 different bullets just from Nosler on the shelf, 17 from Speer, 12 from Hornady, 10 from Barnes, 16 from Sierra & 8 from Combined Technology. Here is your list of bullets. Now you are really confused!
BARNES Bullets (10)
XLC Coated, .30 caliber 150 gr., Boattail
XLC Coated, .30 caliber 165 gr., Boattail
XLC Coated, .30 caliber 180 gr., Spitzer
X-Bullets, .30 caliber 150 gr., Spitzer
X-Bullets, .30 caliber 150 gr., Boattail
X-Bullets, .30-30 caliber 150 gr., FN
X-Bullets, .30 caliber 165 gr., Spitzer
X-Bullets, .30 caliber 165 gr., Boattail
X-Bullets, .30 caliber 180 gr., Spitzer
X-Bullets, .30 caliber 180 gr., Boattail
COMBINED TECHNOLOGY Bullets
Ballistic Silvertip .30 caliber 150 grain BT .308"
Partition Gold 30 caliber 150 grain Spitzer .308"
Fail Safe 30 caliber 150 grain HP .308"
Fail Safe 30 caliber 165 grain HP .308"
Ballistic Silvertip .30 caliber 168 grain BT .308"
Ballistic Silvertip .30 caliber 180 grain BT .308"
Partition Gold 30 caliber 180 grain Spitzer .308"
Fail Safe 30 caliber 180 grain HP .308"
HORNADY Bullets (12)
Hornady 30 Caliber 150 grain SP .308"
Hornady 30 Caliber 150 grain BTSP .308"
Hornady 30-30 Caliber 150 grain RN .308"
Hornady 30 Caliber 165 grain SP .308"
Hornady 30 Caliber 165 grain BTSP .308"
Hornady 30 Caliber 168 grain A-Max .308"
Hornady 30 Cal. 168 grain BTHP Moly Match .308"
Hornady 30 Caliber 170 grain FP .308"
Hornady 30 Caliber 178 grain A-Max .308"
Hornady 30 Caliber 180 grain SP .308"
Hornady 30 Caliber 180 grain BTSP .308"
Hornady 30 Caliber 180 grain RN .308"
NOSLER Bullets (11)
Partition .30 caliber 150 grain Spitzer .308"
BTH 30 caliber 150 grain Spitzer .308"
Custom Competition 30 caliber 155 grain HPBT .308”
Partition .30 caliber 165 grain Spitzer .308"
BTH 30 caliber 165 grain Spitzer .308"
Custom Competition 30 caliber 168 grain HPBT .308”
Partition 30 caliber 170 grain Round Nose 30-30 .308"
Partition 30 caliber 180 grain Spitzer .308"
Partition .30 caliber 180 grain Prot. Point .308"
Partition .30 caliber 180 grain Prot. Point .308"
BTH 30 caliber 180 grain Spitzer .308"
SIERRA Bullets (16)
Sierra 30 Caliber 30-30 150 grain FP .308"
Sierra 30 Caliber 150 grain FMJ BT .308"
Sierra 30 Caliber 150 grain Spitzer BT .308"
Sierra 30 Caliber 150 grain Spitzer .308"
Sierra 30 Caliber 150 grain RN .308"
Sierra 30 Caliber 150 grain HPBT .308"
Sierra 30 Caliber 155 grain HPBT .308"
Sierra 30 Caliber 165 grain HPBT .308"
Sierra 30 Caliber 165 grain Spitzer BT .308"
Sierra 30 Caliber 168 grain HPBT .308"
Sierra 30 Caliber 168 grain HPBT Moly .308"
Sierra 30 Caliber 175 grain HPBT .308"
Sierra 30 Caliber 180 grain HPBT .308"
Sierra 30 Caliber 180 grain Spitzer BT .308”
Sierra 30 Caliber 180 grain Spitzer .308"
Sierra 30 Caliber 180 grain RN .308"
SPEER Bullets (17)
308-150 GR. Flat Nose Soft Point 30-30
308-150 GR. Round Nose Soft Point
308-150 GR. Spitzer Soft Point
308-150 GR. Spitzer FMJ BT
308-150 GR. Spitzer Soft Point BT
308-150 GR. Mag-Tip Soft Point
308-150 GR. Grand Slam
308-165 GR. Spitzer Soft Point
308-165 GR. Spitzer Soft Point BT
308-165 GR. Grand Slam
308-168 GR. HP-Match BT
308-170 GR. Flat Nose Soft Point 30-30
308-180 GR. Round Nose Soft Point
308-180 GR. Spitzer Soft Point
308-180 GR. Spitzer Soft Point BT
308-180 GR. Mag-Tip Soft Point
308-180 GR. Grand Slam
We now have a total of 74 bullets to narrow down to one bullet that we are going to use for our whitetail and possible hog hunt. We know that all of the 150gr bullets have an SD of .226, the 165gr have an SD of .248, the 168gr have an SD of .253, the 170gr SD is .256 and the 180gr bullets max out at .271. What does this tell us. In a very short answer, the heavier the weight of a bullet in the same caliber the higher the SD, this in turn should give us more penetration of our bullet on an given animal. While this is generally very true we have to also look at a few variables. One such variable is somewhat easier to look at and that is the specific application that the manufacturer made a certain bullet for. In a 30 caliber bullet various manufacturers make 150gr & 170gr bullets for the 30-30. This cartridge is a fairly low velocity cartridge when compaired to a .308 Winchester using the same weight bullet. Being that it was designed for the velocity range of the 30-30 it was designed to expand and hold together at a maximum velocity of around 2300fps. If we use this bullet in our .308 at a velocity of 2700fps this bullet will in most cases come apart on impact unless the game is far enough away for the velocity to drop into it’s designed range. This being the case we take the bullets designed for the 30-30 off of our list.
This now brings our list down to 66. What else can we use to reduce the number of our available bullets to narrow our choice.
We will not use a bullet designed for Match or Competition as all manufacturers recommend that their match/competition bullets not be used for hunting game animals. I am not going to get into any arguments as to the successes that individuals have with them as these bullets “Are not designed for hunting game animals.” This equals 15 bullets on our list.
We now have only 51 bullets to choose from. Lets go and take off any round nose bullets as we would like to use a bullet that will be able to maintain it’s velocity as high as possible out to our 250 yard range.
That brings us down another 5 for a total left of 46 bullets on our list. What else can we use to bring it down a bit more. In your reading you have read that some of the bullets on your list are a very popular choice in the higher velocity 30 caliber cartridges such as the “Magnum” rounds and that those bullets were being used on much larger animals such as Elk, Moose, Caribou, Grizzly and Bison. Well we don’t need this heavily constructed bullet do we with the rifle we are using and the game we are after so lets take off those that we feel are for much larger game.
There is another 11 to take off bringing us down to only 35 bullets to make our selection from. Just for the sake of removing some bullets we decide that we want a bullet that has a Ballistic Coefficient of no less than .410 so that we can retain our velocity well out to our range of 250 yards.
Now we are down to 29 bullets, being that we are looking at the possibility of getting a wild hog in our sights we know that we would like to insure that we are using enough bullet even at a close range and we have read that some 150gr bullets may not provide the penetration that we would like so we decide to take off any that are 150gr.
Nine more bite the proverbial dust and we now have only 20 to make our selection from, what do we do now? Let’s take a hard look at what we are hunting, in many cases with reading and communicating with other hunters and what they use we have come to a fairly educated decision based on the cartridge that we are using and the game we are after we really don’t need the heavier weight of the 180 grain bullet so we will take them off of our list.
This leaves us with a selection of (11) - 165 & 168 grain bullets to choose from, this list is as follows:
XLC Coated, .30 caliber 165 gr., Boattail
X-Bullets, .30 caliber 165 gr., Spitzer
X-Bullets, .30 caliber 165 gr., Boattail
Ballistic Silvertip .30 caliber 168 grain BT .308"
Hornady 30 Caliber 165 grain SP .308"
Hornady 30 Caliber 165 grain BTSP .308"
Partition .30 caliber 165 grain Spitzer .308"
BTH 30 caliber 165 grain Spitzer .308"
Sierra 30 Caliber 165 grain Spitzer BT .308"
308-165 GR. Spitzer Soft Point
308-165 GR. Spitzer Soft Point BT
Our bullets weighing 165 grain have an SD of .248 and our lone 168 grain has an SD of .253, according to the charts this fall within the recommended range of Sectional Density for our game that we are hunting so what do we look at now. Given that we have looked at the reloading data for our cartridge we know that we will be able to load our bullet to a velocity of 2700fps. We know that the deer we are after is a light skinned animal and will not provide much ”Material Resistance” to our bullet but at the same time if we happen to get a shot at a hog the ”Material Resistance” will be much higher. Thus we need to know or make an informed decision as to which bullet we may decide to use. This is where the past results of others plays a large role, along with the re*****tion of the manufacturer. In most cases any of the above bullets will, with proper shot placement put venison and/or pork in your freezer, with your shot not being over 250 yards the difference in trajectory between a boat tail and a flat base bullet is so slight you would not realize the difference. Out of the above selection I will tell you this, the Barnes XLC and X-Bullet along with the Nosler Partition will without a doubt provide more penetration than any of the others. The Barnes bullets will expand the least of any of them also there have been cases of boat tail bullets loosing their core, on a deer this may not have any bearing on the final outcome but on a large hog it very well could. The last thing that you need to do is to shoot them to see which one meets your accuracy requirements.
With my past experience as well as that of many others I will give you my choices in the order of my preference.
1) Nosler 165 grain Partition – Very accurate Premium bullet in my rifle, taken hog, whitetail, caribou & black bear with this bullet with no bullet failures.
2) Combined Technologies Ballistic Silvertip 168 grain – hog & deer, sub MOA out to 300 yards with no bullet failures.
3) Barnes XLC Coated 165 grain Boat tail, not as accurate as the Nosler Partition but will out penetrate the Partition any day, probably due to the fact that it will not expand near to what the partition will.
4) Nosler 165 grain Ballistic Tip (Hunting) – As with the CT Ballistic Silvertip, Sub MOA and not a failure so far. Although I have read and heard reports that most of the Hunting BTs .308 and under may cause extreme tissue destruction at impact velocities over 2600 – 2700fps. I have roughly calculated that the highest impact velocity that I have reached was around 2550fps.
On the other hand, some of those that we took off our list would make very good bullets
for our hunt as they will penetrate in some cases even better than a standard 165 grain bullet. These are the 150 gr. Nosler Partition, the 150gr. Speer Grand Slams, the 150 gr. Failsafes and the 150 gr. Barnes variations. Hopefully you are not more confused at this point and you understand that there is more to a bullets ability to penetrate than it’s numerical Sectional Density.
In closing just think of this; You have a 500 pound hog that will stand still, will jump up alive after being dispatched to allow you to shoot it again and again with all of the bullets available to you with varying SDs, construction, shape, material, weight & caliber. How well will your bullet work at the following velocities. 100fps, 500fps, 1000fps, 1500fps, 2000fps, 2500fps, 3000fps, 3500fps & 4000fps. Plug in any velocity you wish, your hog is there for your testing and it doesn’t change. What bullet will you end up using? I’ll bet you won’t base your future selections based on Sectional Density any more!!! |
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