Bullet Penetration

Bullet Penetration

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SD is usually given for unfired bullets, but it's the SD of fired slugs that usually matters. Richard Mann

Many shooters believe that the sectional density of an unfired bullet predicts penetration, but for the vast majority of bullets we use, that just isn't true

By
Richard Mann
|

Published Feb 3, 2022 12:18 PM

We need to talk about the sectional density (SD) of an unfired bullet. Why? Because you’ve been led to believe that it matters. And it doesn’t really. Not for most of us. I know this may come as a shock, given that many shooters believe that the higher a bullet’s initial SD, the deeper it will penetrate. Bullet catalogs list unfired SD as an important specification, after all, and the supposed correlation of SD and bullet penetration is all over Internet chat forums and even articles like this one. But if you just think about a bullet made of cheese, you’ll see what I’m getting at.
First, let’s define the term: SD is the ratio of a bullet’s weight to its diameter. It’s calculated by dividing the bullet’s weight in grains by 7,000, which gives you it’s grains per pound, and then dividing that by the bullet’s diameter, squared. With solids or bullets that do not deform, SD can be representative of penetration. But here’s the key: Most of us use bullets that do deform, and with these projectiles, the SD of the unfired bullet means very little. Rather, it’s the SD of the mushroomed bullet (which isn’t given in catalogs or much talked about) that truly influences penetration.
To better understand SD, you have to realize that its calculation does not consider what bullets are made of or how they’re constructed. SD may seem to reflect how hard or tough a bullet is, but it doesn’t. Which brings us to the bullet made of cheese. Consider one bullet made of lead and another one made of cheddar. If both weigh the same and both have the same diameter or caliber, their sectional densities are identical. Unless you’re hunting field mice, the cheddar cheese bullet is as useless as a lasso on a whale.
More practically, let’s look at two bullets commonly used in the .30/30 Winchester. A 150-grain .30-caliber Barnes Flat-Nose Triple Shock has the same SD as a 150-grain .30-caliber Winchester Power Point. If the SD of the unfired bullet was such a good indicator of penetration, these bullets would penetrate to the same depth. But they don’t.
The Barnes bullet deforms with a frontal diameter of 0.600 and has a recovered weight of 150 grains. The Power Point bullet deforms with a frontal diameter of 0.634 and has a recovered weight of 134 grains. Impacting at the same velocity, the Barnes bullet will penetrate about 20 percent deeper. It’s not the SD of the unfired bullet that influences penetration, it is the terminal sectional density (TSD) of the deformed/expanded bullet that matters. The Barnes bullet has a TSD of 0.060 as compared to the Power Point’s TSD of 0.048.
The TSD of a bullet—its recovered weight divided by the square of its recovered diameter—is highly influenced by impact velocity and will vary depending on how fast it is traveling when it hits something. After only a couple inches of penetration, bullets fully mushroom and establish their terminal shape. The remaining 90 percent of the bullet’s penetration journey is then dictated by the mushroomed bullet’s TSD and remaining velocity. Because bullets are constructed differently, made of different materials, and react differently to impact, you cannot reliably predict penetration based on the SD of the unfired bullet. TSD, on the other hand, is a very good indicator of penetration potential. But ironically, to calculate a bullet’s TSD you must recover it, weigh it, and measure it. If you can do that, you already know how far it penetrated.
TSD is the heart of terminal performance when it comes to bullets that deform or expand. It is the reason some hunters prefer a certain type of bullet to another. Just look at the three .30-caliber, 150-grain bullets from Nosler pictured below. From left to right, they are a Ballistic Tip, an AccuBond, and a Partition. They all have the same SD, but their TSDs, when captured in ordnance gelatin at about 2600 fps, are 0.045, 0.051, and 0.063 respectively. And these TSDs correlate very well to their respective penetration depths of 17.5, 21.0, and 22.5 inches.
Hunters will sometimes use the SD of unfired bullets when trying to decide on the best bullet weight to use for a particular application. It’s generally thought that heavier bullets of a given caliber penetrate deeper, due to their higher SD. This seems to make sense, but it’s simply not always the case. This is partly because you must consider the higher velocity of the lighter bullet. And partly because, not all bullets of the same type are designed to react to impact in exactly the same way.
Look at thee three .30-caliber Swift Scirocco bullets below. The 150-grain bullet (left) has an SD of 0.226, the 165-grain bullet (center) has an SD of 0.248, and the 180-grain bullet an SD of 0.271. When fired at standard .300 Winchester Magnum velocities for each bullet weight, and captured at 100 yards in ordnance gelatin, these bullets all penetrate to an almost identical depth. Why? It’s partly because the lighter the bullet, the higher the impact velocity. But it’s also partly because, even though these are all the same type of bullet, they do not react to impact exactly the same. The lighter bullets are designed to retain more weight to help them cope with the higher impact velocities they will experience.
This proves yet again that, unless you’re talking about non-deforming bullets or solids, the SD of the unfired bullet does not provide us with much useable information regarding penetration. How a bullet reacts when impacting an animal is influenced by many factors; it is a chaotic event, and we can only make an educated guess at the outcome. One thing, however, is for sure: The construction, design, and impact velocity of bullets are far more important than their unfired SD.

Richard Mann was born and raised in West Virginia and has hunted from the Montana mountains to the green hills of Africa. In 2015, Mann began contributing to Field & Stream to cover guns, ammunition, ballistics, and hunting. In 2022, he was named as the brand’s Shooting Editor.

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By

Bob Campbell

Published on March 17, 2022
in Safety and Training

Wallboard doesn’t stop most projectiles.
Lightweight .223 bullets stopped short in wallboard, and many fragmented.
The Fiocchi 40-grain V-Max is an inoffensive home defense round.
The author shot this old hardened board with a .45, left; 9mm, center; and .22LR, right.
The 12-gauge shotgun is a formidable weapon that destroys home material easily.
We shot this old hardened board with the .45 ACP 230-grain FMJ loading.
This small well house was purchased at a yard sale. The construction is similar to home material, and the boards were treated to resist the elements.
A lot of testing goes into this type of research.
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About the Author:
Bob Campbell
Bob Campbell’s primary qualification is a lifelong love of firearms, writing, and scholarship. He holds a degree in Criminal Justice but is an autodidact in matters important to his read ... ers. Campbell considers unarmed skills the first line of defense and the handgun the last resort. (He gets it honest- his uncle Jerry Campbell is in the Boxer’s Hall of Fame.)

Campbell has authored well over 6,000 articles columns and reviews and fourteen books for major publishers including Gun Digest, Skyhorse and Paladin Press. Campbell served as a peace officer and security professional and has made hundreds of arrests and been injured on the job more than once.

He has written curriculum on the university level, served as a lead missionary, and is desperately in love with Joyce. He is training his grandchildren not to be snowflakes. At an age when many are thinking of retirement, Bob is working a 60-hour week and awaits being taken up in a whirlwind many years in the future.

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Home » Safety and Training » Throwback Thursday: Bullet Penetration in Home Environments
When choosing a home defense handgun, rifle, or shotgun, the gauge, caliber, and load are important considerations. If you are in a crowded area, this is important. If you reside in a rural area or the home is isolated, bullet penetration of building material doesn’t mean as much. Or does it? There may be family members in other parts of the home, and the shots cannot be called back once they are sent out.
The primary goal of a personal defense load is to penetrate the adversary’s body, do damage to vital organs and cause a shutdown of the pressurized system. Only blood loss will cause this shutdown. The responsibility of the home defender is to strike the target.
Firing when you do not have a reasonable expectation of hitting the target isn’t a responsible action. Being human, you may miss, and you should know what to expect when a bullet connects with common building materials.
There is an interest expressed by defensive shooters in the use of cover, and I felt I should take that track as well in my testing. What types of building material will give the homeowner cover if they are under fire? After all, bullets go both ways.
Hardening a home against gunfire is a daunting proposition. If you elect to pour concrete between the walls, be certain the foundation will hold the weight. Since abandoned homes in a safe area that the owner is willing to allow me to shoot to pieces are rare, I constructed simulated walls and fabricated test objects.
Next, I fired a few hundred rounds of ammunition and destroyed a truckload of structures. In the end, I discovered loads that are very attractive choices for home defense.
You cannot sacrifice bullet penetration by choosing loads that blow up on heavy leather jackets or fail to penetrate to the vitals, but neither do you wish to use a magnum hunting load in your handgun . The primary focus was on the handgun, as the handgun is always with us, but I also studied rifle and shotgun performance.
The problem of overpenetration in the home or apartment has been discussed many times. The fear of a bullet carrying far past the area of a gun battle is real. Wounds received by innocent persons happening to be in the wrong place at the wrong time are not uncommon. However, most of these occur in the open or on the street and are a result of missed shots rather than overpenetration. The great majority is the result of actions by our protein-fed, ex-con criminal class rather than honest citizens. Incidents involving homeowners firing through walls or a bullet exiting the home are less common. The majority of armed citizens are mature, conscientious individuals. I looked at the whole picture and tested ammunition from several angles.
Homes differ in construction, and at best, a general understanding of home penetration parameters was gained. Responsibility for every round fired is a concern and so is safety from a bad guy’s bullet.
The first solution to the danger of an overpenetrating bullet exiting a home is to hit what you are shooting at, as I keep repeating. Carefully place a hollowpoint bullet into the assailant to let it expand properly and there will be no overpenetration problem.
Know the distance between one occupied structure and the other. Since we usually fire at an angle slightly up or down and seldom perpendicular to the target, the chances are the bullet will come to rest in something before it reaches an occupied home. A studio apartment with thin walls is another matter.
Hopefully, the burglar will present himself to your view and enfilade fire will get his attention. If he is in defilade, however, and under cover, you may need the ability to penetrate cover rather than limiting bullet penetration.
There have been ridiculous cinema depictions of felons using a chair or mattress for cover. Bullets zip through these constructs, but some furniture may be heavy enough to turn a bullet. Will you be engaging in wall stitching in the home? Are there bullets that expand well in a body and stop short in home material?
Many high-velocity hollowpoint bullets will expand in the body, true, but when they hit the wall the bullet nose plugs and they penetrate like a full-metal-jacket bullet. I noted that every test program in the popular press was inconsistent, with every author having a different idea, including some of the work I have done — after all, I learn things as I go along.
One of the considerations never mentioned in the popular press is the existence of firewalls between walls in apartment complexes. (Older buildings may not have them, but let’s hope you live in a dwelling with firewalls.)
Stairwells and common hallways are an important consideration for the defensive handgunner. Another consideration is the difference between high-speed, light bullets and heavy, low-speed bullets.
Drag is a slowing force that seems to work more quickly on fast-moving, light bullets. Heavy bullets, even when moving slower than light bullets, tend to penetrate more, as is the case with the 147-grain, 1,000-feet-per-second 9mm Luger versus the 115-grain, 1,200 fps 9mm and the 230-grain, 850 fps .45 ACP versus the 185-grain, 1,100 fps .45.
I tested a representative sample of handgun rounds against materials including pine board, drywall, cinderblock, and brick and added whatever I could come up with. The information obtained is interesting.
Common sense tells us that a loading by one maker or the other will probably perform similarly to the test load in bullet penetration if not in expansion, such as 115 grains at 1,200 fps in 9mm. Since the nose closes up and the bullet penetrates, a presumption is often made that only velocity and weight really matter, not bullet design. The same goes for 9mm full metal jacket or .45 ACP hardball, with one brand similar to the other.
This is actually true when it comes to FMJ loads but not true with JHP loads. A wide-mouth hollow-point such as the 115-grain JHP used in Buffalo Bore’s +P 9mm load breaks up rather quickly when fired against cinder block and doesn’t penetrate as much in wood as the slightly different Buffalo Bore +P+ load. The +P+ load uses a bonded design. The bullet stays together, as is the intent.
With the development in bullet technology during the past decade or so seeming to focus on bonded-core bullets, we need to carefully appraise our choices. Do we really need a bullet with high penetration against felons behind cover? Are the felons likely to be bundled in heavy clothing?
Bullets with less gelatin penetration are less offensive in home penetration. I stand by the need for greater bullet penetration in defense loads — at least on the level of 10 to 12 inches in water or gelatin — but it is clear that bonded bullets are designed for penetration of sheetmetal and window glass and perform as designed. Whether this is desirable is up to the homeowner.
In this test, the exactness of what we accomplished is respectable, but the great, broad, general conclusions of the test are also important. Judging by my experience in gelatin and water testing, loads that penetrate 12 inches of gelatin or a similar amount of water will not have sufficient energy to be dangerous and penetrate wall material once they penetrate a felon’s body — after all, they should be expanded.
The real difference in safety is hitting the target. Never use roundnose lead or full-metal-jacketed bullets for home defense. While the .45 ACP 230-grain FMJ is an effective anti-personnel load, a JHP bullet is more effective. I did not test ricochet, for safety’s sake, and neither should you. But among the few scares in the program was when RNL bullets bounced a bit when fired into hardwood. Even the .45 ACP, let alone the .38 RNL, will ricochet off a hard gradient.
At one time, pine boards were the testing material for handgun bullets. After all, with RNL or FP lead bullets the FMJ expansion wasn’t a concern but power and bullet penetration were. Around 1900 or so, pine board testing was commonly used by ammunition companies to illustrate the effectiveness of a handgun loading. Today, we might test pine boards in order to understand wall stud vulnerability to bullets.
However, also understand that the studs in a wall will be facing rather than lengthwise, but just the same, this test is valid depending upon the bullet angle. For our tests purposes, the pine boards were set up an inch apart to allow parts of the boards to be separated from the bullet as it flew through the boards.
All boards are not the same, and a quarter-sawed board will
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