Book Chapter 10-Home

Hunting Bullet Metrics

Apply Terminal Performance Truth

AFRICA HUNTER QUEST©

Chapter 10 - HOW IMPACT VELOCITY AFFECTS HUNTING BULLET PERFORMANCE

D: Even though you say this is only going to take 5 minutes, there has been nothing simple concerning the things we have talked about so far. Am I to conclude that the subject of how impact velocity controls bullet performance is more complex than simple?

GG: Conceptually, I don’t think so. Operationally, with all the ifs, thens, and whatabouts, it would take a while.

D: Could we at least start with ‘simple’?

GG: Sure. The simple reason: the impact velocity controls the bullet’s total deformation and rate of deformation.

Donny sat there stunned. The Old Man had just given a reason with only a dozen words.

D: That’s it?

GG: Yep. You wanted it simple.

D: What’s the ‘complex’ part?

GG: The way the bullet deforms, the extent to which it deforms, and the rate at which it deforms determine the wound cavity’s shape, volume, and related dimensions.

D: Is that where the bullet’s generic design comes into play?

GG: Yep. Good call. By deformation I mean how much the bullet compresses to a length shorter than its original length and how big of a mushroom is formed. These properties, as well as the rate at which the bullet length compresses and the rate at which the mushroom is formed, are all related to the bullet’s generic design.

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Each generic bullet design is a premeditated attempt to limit or to control these deformations and rates of deformation…….or not. The net result produces bullets where some designs tend to expand more than penetrate, some tend to penetrate more than expand, and some try to balance these competing objectives. I won’t clutter the conversation with which designs do what and how.

D: Thank God.

GG: Because impact velocity controls the bullet’s deformation, each incremental change in impact velocity produces a corresponding incremental change in the shape and volume of the wound cavity that is produced as well as the penetration length that is achieved. In general, and I mean really general, you can expect two relationships to hold true. First, an increase in impact velocity increases the penetration and increases wound cavity volume. Second, an increase in the rate of deformation decreases the penetration and increases the wound cavity volume.

D: To the point where the bullet’s penetration is zero and it just blows up on impact?

GG: Precisely.

D: I suppose the generality works the other way? That a decrease in impact velocity decreases penetration and decreases the wound cavity volume, and that a decrease in the rate of deformation increases the penetration and decreases the wound cavity volume? That’s why you said some bullets could go from producing a wound cavity that looks like a pissed-off blow fish to one that begins to resemble a finless guppy because there is a significant decrease in impact velocity.

GG: Bingo!

For each generic bullet design and bullet weight, I believe there is an impact velocity range sweet-spot where the bullet is supposed to produce the mix of wound cavity volume and penetration that the manufacturer intended. The generalities I just stated are typically valid only within that impact velocity sweet-spot. The trouble is, most manufacturers don’t precisely state nor provide guidance for what that sweet-spot range is.

D: Precisely state?

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GG: Use the word “recommend”. Or, in terms of guidance, furnish bullet test data that include bullet expansion ratio, wound cavity volume, and penetration at various impact velocities so the hunter can identify if the bullet, fired from his specific chambering, will likely produce the terminal performance he wants at the impact velocities he has identified in his hunting problem definition.

In the book Rifle Bullets for the Hunter by Richard Mann, published by Ballistic Technology, wound cavity volume and penetration data are presented for a 6.5-mm, 130-grain boat-tail spitzer bullet. There are seven impact velocities referenced, ranging from about 1800 to 3200 fps. For each impact velocity, there are test results for wound cavity volume, penetration length, bullet expansion ratio, and bullet weight retained. By bullet expansion ratio, I mean the number used to multiply the caliber diameter to get the measured mushroom diameter. The test medium Mann used is radically different than the 20 % synthetic gel I used, and the test bullets actually formed a wound-cavity hole. The hole was big enough so that it could be filled with water from a graduated cylinder, thus facilitating a direct volume determination.

I believe this data set is truly unique, as I have found no other published reference that includes actual wound cavity volume as a bullet test result. At an impact velocity of about 1800 fps, the bullet expansion ratio was 1.4, the weight retained was 100%, the wound cavity volume was 10 cc, and the penetration was greater than 22 inches. At an impact velocity of 3200 fps, the bullet expansion ratio was 2.1, the weight retained was 90%, the wound cavity volume was 180 cc, and the penetration was 15.3 inches.

D: But the penetration decreased with impact velocity. Didn’t you just say that the penetration should increase?

GG: I said that generality is typically valid only within the bullet’s impact velocity sweet-spot. Mann just presents the data and is silent on its actual interpretation and where the data indicate that sweet-spot was.

In order to assess where that impact velocity sweet-spot might be, I plotted the bullet’s expansion ratio, the wound cavity volume, and penetration versus impact velocity on the same sheet of arithmetic graph paper.

D: (Rolling his eyes) That can’t be done.

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GG: It can if you use three different scales. The point is once the three graphs were superimposed on each other based on impact velocity, it became very clear to me that the blossoming and maximum resultant diameter of the mushroom controlled how the bullet performed in terms of penetration and wound cavity production.

For this bullet and the data set presented, the engineer in me arrived at a ‘reasonable’ conclusion that the impact velocity sweet-spot is from about 2500 to 3200 fps. Within that impact velocity range, the bullet expansion ratio becomes relatively uniform, varying only from 2.1 to 2.3. Because of this relative mushroom uniformity, the penetration incrementally increased with increasing impact velocity from about 12.8 to 15.3 inches, and the wound cavity volume incrementally increased with increasing impact velocity from about 62 to 180 cc.

D: (Smirking) But that’s your interpretation. Once again, you are claiming you are right and not allowing anyone, including the manufacturer, to give the real answer.

GG: My interpretation? You betcha! That, in a nutshell, is the point. Actual data are presented that allowed me to assess the bullet’s likely terminal performance in a variety of hunting scenarios, all on my terms, sweet-spot or manufacturer-recommended impact velocity range be damned.

For example, if I was hunting a big-horn sheep ram, I personally would want to anchor it to keep it from dropping into oblivion or escaping to terrain that I could not possibly access. To me, that terminal performance requirement means I need a bullet that maxes out wound cavity volume at the potential expense of penetration to enhance the odds of actually stopping it or at least limiting travel distance after the shot. If I had a 6.5 Swede that launched this bullet at a muzzle velocity of 2750 fps and my likely shot distance was 300 yards, that would result in an impact velocity of about 2330 fps. My graphs of the test data indicate a rather anemic wound cavity volume of about 50 cc with about 15 inches of penetration. I personally would want a bullet that produced way more wound cavity volume than the test-indicated 50 cc because I don’t need 15 inches of penetration for a big-horn sheep. For that bullet, chambering, and hunting problem definition, the projected impact velocity that results from the rather sedate muzzle velocity of the Swede indicates I would likely need to consider a bullet with a different generic design to max out wound cavity volume.

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On the other hand, if I had a 264 Win mag, the muzzle velocity would be about 3200 fps, putting the expected impact velocity at about 2740 fps at 300 yards. My graphs of Mann’s test data indicate a wound cavity volume at that impact velocity would be about 100 cc, exactly double that which I could expect from the 6.5 Swede. The graphs also indicate a penetration of 13 inches, more than enough for this hunting scenario.

Donnie sat in glum silence. The Old Man used numbers like a Marine sniper used an M40A5. “Damn him.”

GG: In my ideal world, bullet manufacturers would furnish gel test data in Mann’s format for all their bullets. Hunters could then assess and choose bullets based on whatever terminal performance criteria they believed were applicable for each of their hunting scenarios and the chamberings available to them. If I couldn’t get the actual test data, the next best option would be for a manufacturer to specifically recommend an impact velocity range for each bullet that it believes produces its intended terminal performance.

Your astute and correct observation about the bass-ackwards penetration versus impact velocity relationship embedded in Mann’s data set brings up a fundamentally important point and concepts. Outside of a bullet’s impact velocity sweet-spot, terminal performance can be way different from what the referenced generalities predict within the sweet-spot’s impact velocity range. Mann’s data indicate that as impact velocities progressively decrease below the sweet-spot’s lower-bound impact velocity, the penetration typically progressively increases.

D: Increases?!

GG: Yep. That’s because the maximum mushroom diameter progressively decreases with a decrease in impact velocity. The mushroom also blossoms at a slower rate. These two factors combine to result in less total drag on the bullet, which allows it to penetrate more. Because it penetrates more, the wound cavity also tends to increase. That being said, at impact velocities progressively less than the sweet-spot’s lower bound, the wound cavity shape likely begins to progressively look more and more like an eel-snake, ultimately morphing into just a snake. What do you think occurs at impact velocities greater than the sweet-spot’s upper bound?

Donny cyphered on the Geezer’s body language and the tone of his voice. He was being challenged, and an ‘I don’t know’ would be lame and limp.

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D: Guppy changing into a blowfish, then just plain splat.

GG: (Grinning like a Cheshire cat) Bingo! Why?

D: The mushroom would increasingly blossom faster. There would be way more drag way quicker. Both of those factors would decrease penetration. At ‘splat’, the bullet would theoretically disintegrate, producing no penetration nor wound cavity.

GG: Excellent appraisal. What do you think happens to wound cavity volume as impact velocities progressively increase above the sweet-spot maximum?

D: I would expect it to progressively decrease because of rapidly decreasing penetration, finally getting to zero at ‘splat’.

GG: Yep. Good call.

Some manufacturers appear to hint at guidance or a recommended impact velocity range. On one website I know of, there are pictures of three deformed bullets with a velocity shown underneath each one. Velocities are 1800, 2400, and 3050 fps. The title above all three bullet pictures is “Optimum Bullet Performance”. Does this pass muster as a recommendation for impact velocity range?

Donny sensed an ambush. He wanted to say yes, but he was pretty sure the old coot was going to toast him if he said so. He was now pretty sure that the answer would wind up being “no”. He could say “no”, but was sure he would be asked “why”. He reverted to the fallback and embarrassing answer.

D: I don’t know.

GG: Neither do I.

I suppose the manufacturer thought the presentation was a good-faith portrayal of such a recommendation. However, there were no key supporting details to drive that intent home. The bullet weight and caliber weren’t identified. That would have been fine if mushroom expansion ratios were furnished, but they weren’t. No expanded mushroom diameters. No deformed lengths. No indication of whether the velocities shown were impact or at the muzzle. No mention of impact media. Gel? Newsprint? An actual animal? No penetration lengths associated with each velocity. No retained bullet weights. Regrettably, no wound cavity data. Even

with all that basic information being left out, if the title had read ‘Optimum Performance Velocity Range’, I would be inclined to say yes.

“Holy $#!,” thought Donny. “Anal retentive to the max! Unexpected restraint and PC.”

GG: On another manufacturer’s website, there is a picture of an un- deformed 30-caliber, 180-grain bullet. Next to it are shown five deformed bullets, each with a velocity, weight retained, and an actual mushroom diameter. The velocities shown are 1850, 2100, 2500, 2700, and 3325 fps. Does this pass muster as a recommendation for an impact velocity range?

D: Based on what you just said, the manufacturer probably thought that was what was being provided. I take it that the word ‘recommend’ was nowhere to be found. From your description, the velocities and media weren’t defined, and no penetration lengths or deformed lengths were furnished. Again, no wound cavity volumes. Now that I kindly know what information could be presented, the portrayal really doesn’t give me the warm and fuzzies.

In comparison to the examples you have given, just a picture of an animal next to a bullet is kinda lame. With all these examples there is no specified guidance. I guess folks would be on their own trying to figure out if the indicated bullet would work to their satisfaction in their specific hunting scenario using their specific chambering.

GG: The operative word is ‘guess’. And yes, we are on our own. There is another factor that affects a bullet’s deformation. What do you suppose it is?

Donny sat and stared at the Old Man. Damned if the Geezer hadn’t sucked him into thinking on his own. He was being given clues in the form of concepts, and the concepts needed to be manipulated and arranged like puzzle pieces to see the picture. Now all of a sudden, a response of “I don’t know” was at least lazy, even lame. “I don’t know” with the Old Man was always an appropriate response, and sometimes it was the only appropriate response. But Donny now knew he was being given the actual puzzle pieces to fit together, then asked to find other puzzle pieces that fit in the partial picture that was being crafted. In that situation, giving a wrong answer was actually better than “I don’t know” because it likely precipitated a beneficial conversation about why the puzzle piece he selected didn’t fit.

So, what were the conceptual clues? Bullet deformation controlled by impact velocity. The constant undercurrent of “Africa is different”. How were they linked? Donny decided to take a chance.

D: Aim point on the animal?

GG: Bingo!!! Why is that?

D: The high percentage shot is on the shoulder. The bullet will deform more than if the shot was on the lungs because there is more muscle mass to penetrate and the bones are likely stouter. The muscle may even be denser.

GG: So would it be reasonable to say that a bullet that works well on a lung shot would work well on the shoulder?

“Crafty bastard,” thought Donny.

D: Depends on the impact velocity.

GG burst out with a belly laugh comparable to the blast from a 375 H&H with a muzzle brake.

GG: Well done, Pilgrim! But let me ask you this: do you think your 130-grainer will perform well enough to take a kudu?

D: A month ago I would have said absolutely. I was more worried about my chambering than my ammo based on my actual hunting experiences. I considered the ammo to be first-rate, based on its demonstrated performance on deer. Now that I have talked to you and read some of Robertson’s book, I am not so sure. The manufacturer of the bullet implies with a picture that it is good for elk. Maybe, maybe not, maybe depends on impact velocity and aim point. Robertson doesn’t mention the manufacturer at all when he talks about bullets that he recommends for a kudu. Not only that, he thinks that even a .277, 150-grainer is marginal. Regardless of shot aim point, you have just said impact velocity in conjunction with the 130-grainer’s generic design controls deformation, and that deformation controls how my bullet will actually perform. I’m not even sure of what my bullet’s generic design might be. I’m kindly lost.

GG: Don’t feel like the Lone Ranger. I felt basically the same way until I did my gel testing. I was relying on what practitioners like Foster and Robertson had

to say about bullet performance because I believed they are sufficiently divested from applicable special interests. Based on what I have experienced, you first need to decide what your terminal performance objective is. That locks down the important things you want your bullet to do. The hard part is then trying to sort out which generic bullet will do what you want at the impact velocities you have determined in your hunting problem definition. I am certainly no expert at any of this, but I would be pleased to share what I’ve learned.

D: How about we begin right now?