SD Ammo, SD, and what it means and why

This is a discussion on SD Ammo, SD, and what it means and why within the Defensive Ammunition & Ballistics forums, part of the Defensive Carry Discussions category; Seems like lately, there been alot of talkin about lighter ammo vs heavy ammo. As most know, I am a fan of heavy for caliber ...

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Thread: SD Ammo, SD, and what it means and why

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    VIP Member Array glockman10mm's Avatar
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    SD Ammo, SD, and what it means and why

    Seems like lately, there been alot of talkin about lighter ammo vs heavy ammo. As most know, I am a fan of heavy for caliber bullets for all uses. So I figured I'd take a shot at explaining some things about sectional density, weight, bullet construction, and how it becomes a factor in ammo selection.

    Now, I know there are many here that prefer lighter, and I'm not trying to change anyones mind. This is meant to be helpful to those that are new to the whole gun thing, and give them the basics without ging to deep.

    So, while I sit here and sip this Kentucky Bourbon, I'll begin. Please excuse me if my sppeech gets a little slurred, or I start to wander off topic.

    When faced with the choice between lighter, faster or heavier and slower rounds for defensive use for a given caliber, I prefer and recommend the heavier option. While there is much debate and a good arguement for both sides, the lighter and faster has some drawbacks, except for the 125 grn 357 magnum loadings, which I will address later.

    A bullet is made lighter for two reasons; first to give it speed, and two , give it flatter trajectory, which the increased speed accomplishes. Since a flatter trajectory is a mute point for SD purposes, we are left with speed. So what does the increased speed accomplish for us?

    By increasing speed, we increase impact in the flesh of what is being shot. This is usually defined as energy, or how much power the bullet hits with. Now, this is also useful for penetration. Penetration is of utmost importance for a bullet fired with the intent of killing an animal or stopping a threat. This is because it much reach the vital organs to damage and "shut down" that living thing that is being shot. Anything that does not penetrate deeply enough to do this results in nothing more than a nasty sub surface "flesh wound", which may hurt like hell, but does nothing to shut the bodies system off.

    For the most part, lighter faster bullets work well under perfect circumstances. They depend on three things to work properly, and that is speed, penetration and expansion.
    The problem is, that anything that hinders the speed of the lighter bullets, also cancels out, or greatly inhibits the last two important things. Without speed, the bullet may not expand as designed, and worse yet, may not properly penetrate. A bullet that does not properly penetrate to the vitals is the stuff horror flicks portray, when shot over and over, the mask wearing, knife welding boogie man will not stop.

    This is due in part, because, when we have a lighter bullet, we are giving up "sectional density". Sectional density is the amount of material in a bullet from the tip to the base, or in other words, the length of the bullet.

    This is very important because it adds weight to the back of the bullet, giving it weight and momentum. Think of a bullet expanding, starting from the cylindrical shape it has when fired, hitting a medium, and beginning to mushroom. As the bullet becomes larger on the front end, it builds resistance as it expands from encountered tissue and fluid that it pases through. The more it expands, the more resisitance it encounters, therby slowing its forward momentum until it stops

    A heavier bullet of the same caliber, will have more weight in the shank (back of bullet behind the expansion) resulting from a more elongated profile,(Sectional density). The combination of weight and mass left in the heavier bullet give it penetrating power that is not dependent on the speed that the lighter bullet needs to accomplish the same goal.

    So, they both will do the same job, but one is less variable dependent than the others. What does this mean? Simply that if perfect conditions are not present, at least we know we will still get the single most important performance goal met.

    It is called penetration. The bullet reaching the vital organs.

    I am not saying that lightweight faster bullets will not work, or be a failure. There are many documented cases of great success with light, fast bullets working.

    One of the greatest success stories of the defensive cartridge community is the legendary 357 magnum in the lighter 125 weight loading, exploding out of a 4 inch barrel at 1450fps! So how can this be so? If we look at the 357 mag sectional density, we can see that for a 125 grain bullet, it is .140 . So how does that stack up against your favorite 124 grn 9mm loading? A 124 grn 9mm bullet has a SD of .141
    So you think your 9mm in that load stacks up right? Well heres where it gets tricky, and the 9mm loses. Top velocity for the 124 grn 9mm is 1175 fps. Top speed for the 357 mag in the 125 grn load is 1450fps. All things being equal, speed is the difference here, and the 9mm will never be a 357 magnum simply due to lack of case capacity. ( this info obtained from the Hornady 4th Edition Reloading Handbook)

    To be more fair, if we compare the same cartridge, and stay with the 9mm, the 124 weight bullet with an SD of .141, is pretty short in length compared to a 147 weight 9mm with a SD of .167
    Comparing the 9mm 147 weight bullet to the 40 S&W is also interesting. Once again, the 147 weight 9mm has a SD of( .167). By comparison, the 155 weight 40 is(.138) and the 180 weight is (.160)

    You really want an eye opener? The SD of the venerable 45acp in its manstopping 230 grain loading even loses in the SD comparison to the big ole 45 bullet. It has an SD of .162!

    So, to bring it all together, what Im saying is that you either have to have weight to reliably penetrate, which is gained easily in the larger calibers, say 44-45, or you must have enough Sectional density to provide the necessary forward momentum in the smaller calibers.

    In this way, your load is not variable dependent. While nothing is fullproof, the heavier bullet will be a more consistant penetrator in flesh over a wider range of conditions. And proper penetration is a maker or breaker of stopping power.

    Hope this is helpful to someone....now if I can get Mrs Gman to fill my glass.
    Ignorance is a long way from stupid, but left unchecked, can get there real fast.

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    VIP Member Array Majorlk's Avatar
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    Great read, but one nit to pick ...

    Since a flatter trajectory is a mute point for SD purposes,
    The word is moot, not mute. :)
    An armed society is a polite society. Manners are good when one may have to back up his acts with his life. - Robert A. Heinlein

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    VIP Member Array shockwave's Avatar
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    F = m * a

    Basic physics teaches that force equals mass times acceleration. Same as e = mc2. The variables can be set to equal desired outcomes.

    A faster lighter bullet will carry the same force as a slower, heavier one.

    I don't pay much attention to this argument. For me, it's all about measured, proven expansion. The Corbon DPX round consistently produces the perfect, star-shaped ragged petal projectile I look for in a SD cartridge.
    "It may seem difficult at first, but everything is difficult at first."

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    "Simply that if perfect conditions are not present, at least we know we will still get the single most important performance goal met.

    It is called penetration. The bullet reaching the vital organs."


    This is the part I like.
    “No possible rapidity of fire can atone for habitual carelessness of aim with the first shot.”

    Theodore Roosevelt, The Wilderness Hunter, 1893

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    Quote Originally Posted by shockwave View Post
    F = m * a

    Basic physics teaches that force equals mass times acceleration. Same as e = mc2. The variables can be set to equal desired outcomes.

    A faster lighter bullet will carry the same force as a slower, heavier one.

    I don't pay much attention to this argument. For me, it's all about measured, proven expansion. The Corbon DPX round consistently produces the perfect, star-shaped ragged petal projectile I look for in a SD cartridge.
    Pesky physics always seem to get in the way.

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    VIP Member Array smolck's Avatar
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    Quote Originally Posted by glockman10mm View Post
    One of the greatest success stories of the defensive cartridge community is the legendary 357 magnum in the lighter 125 weight loading, exploding out of a 4 inch barrel at 1450fps! So how can this be so? If we look at the 357 mag sectional density, we can see that for a 125 grain bullet, it is .140 . So how does that stack up against your favorite 124 grn 9mm loading? A 124 grn 9mm bullet has a SD of .141
    So you think your 9mm in that load stacks up right?
    Here is a contrasting view point from someone more credible than I that covers this topic, albeit with a lighter 115 grain bullet. Start at 8 minutes on the first video and then on to the second. I also find it interesting that in this video (which is clearly an old video) he mentions that deep penetration is the "current emphasis" and why he believes it is a mistake. Pay especially close attention to 1:30 on video two where he mentions that the Cor Bon load in 9mm (115gr) comes almost as close to .357 mag in 125gr as anything. Hmmmm.




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    Distinguished Member Array RevolvingMag's Avatar
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    It was information like this that I was hoping to get in that other thread. I got some fairly decent input, but a lot of it went back to '.45 is better because it's biggest', which was followed with 'nu-uh; .40S&W is better because it's made from unicorn tears and dragon's blood". NOT WHAT I WANTED TO HEAR.

    I wanted help and insight with the physics of it. I know that a smaller, faster object CAN exert as much force as a larger slower object. There just has to be much more speed.

    Thank you for taking the time to clarify. I would buy you a drink- or offer you one of what I have on hand- but I don't think that would ship very well.
    "Rock and load, lock and roll... what's it matter? FIRE!!"

    "Gun control means hitting your target every time."

    Please take everything I say with at least one grain of salt- I am a very sarcastic person with a very dry sense of humor.

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    So, while I sit here and sip this Kentucky Bourbon,

    ...now if I can get Mrs Gman to fill my glass
    This is the part I like.
    Smitty
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    Quote Originally Posted by smolck View Post
    Here is a contrasting view point from someone more credible than I that covers this topic, albeit with a lighter 115 grain bullet. Start at 8 minutes on the first video and then on to the second. I also find it interesting that in this video (which is clearly an old video) he mentions that deep penetration is the "current emphasis" and why he believes it is a mistake. Pay especially close attention to 1:30 on video two where he mentions that the Cor Bon load in 9mm (115gr) comes almost as close to .357 mag in 125gr as anything. Hmmmm.
    You're correct that the video is dated. I won't put words in Mas' mouth, but I suspect his opinions may have evolved with further developments in ammo over the past decade. Almost twenty years ago I sat in his LFI-1 class and heard him tout the advantages of .45 hardball for its ability to penetrate and do damage to bone and organs even without expanding. I don't think the value of that capability has diminished in Mas' mind over the years.
    Smitty
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    Senior Member Array AZ Hawk's Avatar
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    Quote Originally Posted by smolck View Post
    Here is a contrasting view point from someone more credible than I that covers this topic, albeit with a lighter 115 grain bullet. Start at 8 minutes on the first video and then on to the second. I also find it interesting that in this video (which is clearly an old video) he mentions that deep penetration is the "current emphasis" and why he believes it is a mistake. Pay especially close attention to 1:30 on video two where he mentions that the Cor Bon load in 9mm (115gr) comes almost as close to .357 mag in 125gr as anything. Hmmmm.
    Once again...

    Yes, I like Mas Ayoob, and I like a lot of what he says, however, he's always been a bit "out there" on wound ballistics. There are people out there, pun intended, like Dr. Fackler and Dr. Roberts who have devoted a good portion of their lives to the study of wound ballistics and wound ballistics alone, and I tend to agree more with their scientific findings through the study of ballistic gelatin and gunshot victims.

    I laughed when he said that the 115 gr. CorBon load was the most likely to drop someone with one hit COM because that simply can't be anywhere close to being true unless of course it hits the heart or spinal cord. It simply doesn't have the energy to consistently drop someone with one hit nor would it be wise to shoot someone only once.

    Why do I know this?

    Well, how many times have you heard about people taking multiple shots of .223/5.56 to COM and living?

    Let's look at the differences between LE .223/5.56 ammunition and 9mm 115 gr. CorBon:

    Hornady 5.56 NATO 62 gr. TAP® BARRIER™: Hornady : Law Enforcement | Products | 5.56 NATO TAP® - 5.56 NATO 75 gr. BTHP T2 TAP®

    Muzzle Velocity: 2804 fps
    Energy: 1082 ft-lbs KE
    Barrel: 16"
    Twist Rate: 1 in 9"
    Total Penetration: 13.5"
    Retained Weight: 38.9 gr.

    9mm 115 gr. +P CorBon DPX: http://ammo.ar15.com/project/Ballist...5gr_TAC-XP.pdf

    Muzzle Velocity: 1250 fps
    Energy: 399 ft-lbs KE
    Barrel: 4"
    Total Penetration: 12.7"
    Max Diameter: 0.680"
    Retained Weight: 115.0 gr.

    Now, if 62 gr. 5.56 ammunition does not always stop a threat with a good COM shot, what makes a 115 gr. 9mm bullet with crappy ballistics (compared to the 5.56 round) "most likely to drop someone with one COM hit?"

    I know he wasn't comparing rifle and pistol bullets together because it's silly to do so, but I did so because it further proves my point which is that saying a lightweight round is somehow better because it's faster and more energetic is downright ignorant and/or stupid.
    Move. Shoot. Survive. ― The "Unofficial" Suarez International Doctrine

    “The real man smiles in trouble, gathers strength from distress and grows brave by reflection.” ― Thomas Paine

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    I agree. In the old momentum (MV) vs Kinetic Energy (1/2MV*2) debate, I fall in the former camp. Terminal ballistics will never be an exact science when incapacitating a threat is involved. The body is not an homogeneous substance like gelatin. In fact, different organ densities, sinew, and bone can cause a lighter bullet to deflect, with unintended consequences, even though it can penetrate just as far. Yes, penetration is important, but resistance to deflection is just as important. Deformation of the soft lead core in most HP designs will not only hinder penetration through hard barriers but also cause unpredictable deflection. I use DPX in all calibers for serious work. Although less dense than lead, the 45 cal 185gr copper bullet with a deep HP is unlikely to plug up with leather or denim, and Cor Bon drives it fast while pushing the pressure envelope.
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    Distinguished Member Array 21bubba's Avatar
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    Quote Originally Posted by AZ Hawk View Post
    Once again...

    Yes, I like Mas Ayoob, and I like a lot of what he says, however, he's always been a bit "out there" on wound ballistics. There are people out there, pun intended, like Dr. Fackler and Dr. Roberts who have devoted a good portion of their lives to the study of wound ballistics and wound ballistics alone, and I tend to agree more with their scientific findings through the study of ballistic gelatin and gunshot victims.

    I laughed when he said that the 115 gr. CorBon load was the most likely to drop someone with one hit COM because that simply can't be anywhere close to being true unless of course it hits the heart or spinal cord. It simply doesn't have the energy to consistently drop someone with one hit nor would it be wise to shoot someone only once.

    Why do I know this?

    Well, how many times have you heard about people taking multiple shots of .223/5.56 to COM and living?

    Let's look at the differences between LE .223/5.56 ammunition and 9mm 115 gr. CorBon:

    Hornady 5.56 NATO 62 gr. TAP® BARRIER™: Hornady : Law Enforcement | Products | 5.56 NATO TAP® - 5.56 NATO 75 gr. BTHP T2 TAP®

    Muzzle Velocity: 2804 fps
    Energy: 1082 ft-lbs KE
    Barrel: 16"
    Twist Rate: 1 in 9"
    Total Penetration: 13.5"
    Retained Weight: 38.9 gr.

    9mm 115 gr. +P CorBon DPX: http://ammo.ar15.com/project/Ballist...5gr_TAC-XP.pdf

    Muzzle Velocity: 1250 fps
    Energy: 399 ft-lbs KE
    Barrel: 4"
    Total Penetration: 12.7"
    Max Diameter: 0.680"
    Retained Weight: 115.0 gr.

    Now, if 62 gr. 5.56 ammunition does not always stop a threat with a good COM shot, what makes a 115 gr. 9mm bullet with crappy ballistics (compared to the 5.56 round) "most likely to drop someone with one COM hit?"

    I know he wasn't comparing rifle and pistol bullets together because it's silly to do so, but I did so because it further proves my point which is that saying a lightweight round is somehow better because it's faster and more energetic is downright ignorant and/or stupid.
    Nothing like arguing the extremes trying to make a misguided point.

    Newton, Einstien, yep pretty ignorant and or stupid fellows.

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    I've mentioned before in the other thread that I too like heavier bullets rather than light bullets. However, I carry 185gr DPX, and that's my only exception :) Though, velocity I think is also an important factor. It's clear when you take a 150gr .30 caliber bullet at 2700fps against a 230gr .45 caliber bullet at 850fps. We all know which one wins. Of course, this comparison is a bit much, but it shows that velocity means much more.

    Comparing a 230gr .45 caliber bullet at 850fps vs. a 185gr .45 caliber bullet at 1000fps, I can see the advantages of a heavier bullet. Though, I don't believe energy rating in handgun calibers matter as much as they do in rifle calibers, but usually a lighter/faster moving projectile creates more energy on target in the same caliber. Again, I don't think this really matters in handgun calibers.

    But anyway, we need to remember to rely on the infamous shot placement. It's important to have a good caliber, but it's easy to get caught up miniscule differences in ballistics results that matter very little in real life when shot placement is what matters most.

    That's my "far from expert" opinion. And that's all it is.

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    Senior Member Array AZ Hawk's Avatar
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    Quote Originally Posted by 21bubba View Post
    Nothing like arguing the extremes trying to make a misguided point.

    Newton, Einstien, yep pretty ignorant and or stupid fellows.
    Care to elaborate?
    Move. Shoot. Survive. ― The "Unofficial" Suarez International Doctrine

    “The real man smiles in trouble, gathers strength from distress and grows brave by reflection.” ― Thomas Paine

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    As with everything it's a matter of balance. It's why the big cars typically are more fuel efficient than the roller skates. The big boys have a better ratio of power to size. Similar things have to be taken into consideration with ammunition. Physics is physics after all.

    If a bullet can reach an optimum expansion speed from your gun, and this is most important, then it's effective. Having the heaviest round that can expand makes a bullet better at stopping/killing what you make a good shot with.

    It's why most snubbies should be using lighter weight rounds. They don't have the barrel length to attain the proper expansion velocity of the heavier rounds. Will they stop the target with a good shot? Sure, but a lighter round would most likely do it better. If the shot is slightly off, then the heavier round may not achieve the desired goal whereas the lighter round could.

    It's why I use the 140 grain FTX load from Hornady instead of their 125 grain FTX (CD) load. They expand equally well from a 4" or greater length barrel. The 140 grain round will be slower, but hit harder. For my .357 mag revolver with a 5" barrel, the extra punch of more weight, means better stopping power. You can't go wrong with that.

    Let's not forget that the same weight round from one company may have totally different ballistic characteristics from a second company. Which is why there can't be a simple hard and fast rule in regards to what weight ammo should be used from what caliber in this barrel length range. You need to find the right bullet for your gun. It's a matter of giving yourself the best edge when the time comes.

    There are many good choices for every firearm that is out there now. Find the one that meets your needs, makes you comfortable, and use it. Confidence helps out more than having whatever bullet in your gun after all. You don't want to hesitate if you need to defend yourself/others if the danger is brought to you.

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