The Fact About Sectional Density and Bullet Penetration

We have to discuss in regards to the sectional density (SD) of an unfired bullet. Why? Since you’ve been led to imagine that it issues. And it doesn’t actually. Not for many of us. I do know this may increasingly come as a shock, on condition that many shooters imagine that the upper a bullet’s preliminary SD, the deeper it is going to penetrate. Bullet catalogs listing unfired SD as an essential specification, in spite of everything, and the supposed correlation of SD and bullet penetration is throughout Web chat boards and even articles like this one. However if you happen to simply take into consideration a bullet made from cheese, you’ll see what I’m getting at.

First, let’s outline the time period: 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 supplies you it’s grains per pound, after which dividing that by the bullet’s diameter, squared. With solids or bullets that don’t deform, SD could be consultant of penetration. However right here’s the important thing: Most of us use bullets that do deform, and with these projectiles, the SD of the unfired bullet means little or no. Somewhat, it’s the SD of the mushroomed bullet (which isn’t given in catalogs or a lot talked about) that really influences penetration.

photo illustration explaining sectional density
That is the formulation for the sectional density of an unfired bullet. Richard Mann

To higher perceive SD, it’s a must to understand that its calculation doesn’t think about what bullets are made from or how they’re constructed. SD could appear to mirror how exhausting or robust a bullet is, however it doesn’t. Which brings us to the bullet made from cheese. Contemplate one bullet made from lead and one other one made from cheddar. If each weigh the identical and each have the identical diameter or caliber, their sectional densities are similar. Until you’re searching area mice, the cheddar cheese bullet is as ineffective as a lasso on a whale.

What Actually Issues in Terminal Sectional Density

Extra virtually, let’s take a look at two bullets generally used within the .30/30 Winchester. A 150-grain .30-caliber Barnes Flat-Nostril Triple Shock has the identical SD as a 150-grain .30-caliber Winchester Energy Level. If the SD of the unfired bullet was such a very good indicator of penetration, these bullets would penetrate to the identical depth. However they don’t.

The Barnes bullet deforms with a frontal diameter of 0.600 and has a recovered weight of 150 grains. The Energy Level bullet deforms with a frontal diameter of 0.634 and has a recovered weight of 134 grains. Impacting on the identical velocity, the Barnes bullet will penetrate about 20 p.c deeper. It’s not the SD of the unfired bullet that influences penetration, it’s the terminal sectional density (TSD) of the deformed/expanded bullet that issues. The Barnes bullet has a TSD of 0.060 as in comparison with the Energy Level’s TSD of 0.048.

photo illustration of terminal sectional density
Terminal sectional density makes use of mainly the identical formulation—however after the bullet has been fired, deformed, and recovered. Richard Mann

The TSD of a bullet—its recovered weight divided by the sq. of its recovered diameter—is very influenced by affect velocity and can range relying on how briskly it’s touring when it hits one thing. After solely a pair inches of penetration, bullets totally mushroom and set up their terminal form. The remaining 90 p.c of the bullet’s penetration journey is then dictated by the mushroomed bullet’s TSD and remaining velocity. As a result of bullets are constructed otherwise, made of various supplies, and react otherwise to affect, you can’t reliably predict penetration primarily based on the SD of the unfired bullet. TSD, then again, is an excellent indicator of penetration potential. However mockingly, to calculate a bullet’s TSD it’s essential to get well it, weigh it, and measure it. If you are able to do that, you already know the way far it penetrated.

TSD is the guts of terminal efficiency with regards to bullets that deform or increase. It’s the purpose some hunters favor a sure sort of bullet to a different. Simply take a look at the three .30-caliber, 150-grain bullets from Nosler pictured beneath. From left to proper, they’re a Ballistic Tip, an AccuBond, and a Partition. All of them have the identical SD, however 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 properly to their respective penetration depths of 17.5, 21.0, and 22.5 inches.

photo of three shot and unshot bullets
These three bullets from Nosler have the identical preliminary SD however penetrated fairly otherwise in gel. Richard Mann

Be Cautious Utilizing Preliminary SD to Select a Looking Bullet

Hunters will generally use the SD of unfired bullets when attempting to determine on the most effective bullet weight to make use of for a specific utility. It’s usually thought that heavier bullets of a given caliber penetrate deeper, as a result of their greater SD. This appears to make sense, however it’s merely not at all times the case. That is partly since you should think about the upper velocity of the lighter bullet. And partly as a result of, not all bullets of the identical sort are designed to react to affect in precisely the identical manner.

Have a look at thee three .30-caliber Swift Scirocco bullets beneath. The 150-grain bullet (left) has an SD of 0.226, the 165-grain bullet (heart) has an SD of 0.248, and the 180-grain bullet an SD of 0.271. When fired at commonplace .300 Winchester Magnum velocities for every bullet weight, and captured at 100 yards in ordnance gelatin, these bullets all penetrate to an nearly similar depth. Why? It’s partly as a result of the lighter the bullet, the upper the affect velocity. But it surely’s additionally partly as a result of, regardless that these are all the identical sort of bullet, they don’t react to affect precisely the identical. The lighter bullets are designed to retain extra weight to assist them deal with the upper affect velocities they may expertise.

These Swift Scirocco bullets have very completely different preliminary SDs, but they penetrated nearly the identical. Richard Mann

This proves but once more that, except you’re speaking about non-deforming bullets or solids, the SD of the unfired bullet doesn’t present us with a lot useable info concerning penetration. How a bullet reacts when impacting an animal is influenced by many elements; it’s a chaotic occasion, and we are able to solely make an informed guess on the end result. One factor, nevertheless, is for certain: The development, design, and affect velocity of bullets are way more essential than their unfired SD.

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