Some Thoughts On The 10mm And Energy

Peter Grant, over at Bayou Renaissance Man, has an interesting article on the topic of "Renewed interest in the 10mm Auto as an optimum defensive round." He has a good look at the current and probable future of the evolving criminal threats that private citizens face, and I suggest that you read his article before continuing with my post. I only fault him in one regard: he expresses a common misconception about energy and bullet effectiveness in discussing why the 10mm may be an optimal defensive round. 

    Before I delve into Grant's article, just a little history on the 10mm as a defensive cartridge. After the 1986 FBI shootout in Miami which resulted in two agents being killed and five injured, the FBI drew the lesson that bullet penetration was suboptimal with the firearms it used and so it created standards still used to this day to judge the effectiveness of bullets. These standards drive bullet design to this day, mixing penetration against various materials as well as both maximum and minimum penetration in ballistic gelatin. 

    Because bullet technology was not what it was today, the FBI's quest for a better duty weapon necessarily resulted in it turning to a more powerful round that would be able to ensure both expansion and penetration. That initially was the 10mm, but when the 10mm was found to have too much recoil for its agents, the FBI necessarily started using a down-loaded round; which, in turn, prompted Smith & Wesson to develop the 40 S&W which is, essentially, a shortened 10mm.

    For purposes of my discussion, the key part of the FBI standard is that when shot into ballistic gelatin, penetration was to be no more than 18-inches in order to avoid over penetration--the bullet passing completely through a person. Consequently, hollow-point bullets for police duty use or civilian defense are engineered to open up (mushroom) which acts similar to a drag chute and slows the bullet limiting its penetration. (It also creates a larger wound channel). 

    So, with that in mind, and turning to Grant's article, he makes the common mistake of equating handgun "power" with effectiveness. He writes concerning the 10mm:

    There are those who'll argue that more accurate shot placement would allow the smaller round to perform just as well as a larger one.  That's fine in theory, but when you've got a moving target (the perpetrator) closing on you rapidly, possibly with a weapon in his hand, and you're moving to avoid his attack, and your stress and adrenaline levels are off the charts . . . it's a whole world away from shooting slowly, calmly and without stress on the square range at paper targets that don't move and aren't threatening you.  Some (very few) shooters can perform as well under such extreme stress as they do during training.  Most of us can't.  We should expect to be less accurate - which places more of a burden on the rounds we're shooting to do the best job they can when they hit whatever they hit.

    I know many shooters blindly trust the "official" figures, charts and data, which claim that a quality 9mm hollowpoint will deliver as much expansion, penetration and energy as a bigger round.  They're right, on paper.  However, real life doesn't take place on paper, and drug-addled criminals can and will make a mockery of what "the book" says should happen when they're shot.  I discussed this issue in an article some years ago:

Bullet and cartridge effectiveness for self-defense

    If you didn't read that article at the time, I strongly suggest that you click over there and read it now.  In particular, read the whole thing, not just the beginning.  There's a lot of "meat" in there, and it's all important.  In brief, size (projectile cross-section, bullet weight and overall expansion potential) does matter;  and bigger bores have benefited from the same technological advances that have improved the 9mm.  They've all gotten better than they were.

    In brief, due to the possible need to repel multiple assailants, cartridge capacity is important.  While the 9mm is champion in that regard right now, the 10mm (and its smaller cousin, the .40 S&W) is pretty close on its heels, and all of them usually outperform the venerable .45 ACP.  (For example, comparing full-size models, the 9mm Glock 17 holds 17+1 rounds;  the 10mm Glock 20 and the .40 S&W Glock 22 both hold 15+1 ;  and the .45 ACP Glock 21 holds 13+1.)

    On the other hand, there's the need to hit a potentially hopped-up assailant as hard as possible, to make him cease his attack on you.  The 9mm is no slouch, but it hits less hard than the .40 or .45 - and the 10mm outperforms all of them, if a full-power round is used.

    To understand why the Power = Deadliness equation is not accurate you must remember that, absent the bullet striking and destroying something essential like the heart or the brain, incapacitation of a living target is due to shock either due to hydrostatic shock or the loss of blood, itself largely dependent on the size of the wound channel (both diameter and depth). Unfortunately, no single factor--bullet mass, velocity, energy, momentum, cross section density, bore diameter--can tell you how effective a bullet will be at putting down a living target.

    Because of the properties of the water in living tissue, hydrostatic shock is largely a function of velocity. Turning to Ballistic Studies, a great source of information on terminal ballistics on game animals, it notes that "[h]ydrostatic shock, in bore sizes from .243” up to .338”, begins to lesson at impact velocities below 2600fps and most modern high velocity sporting cartridges including the magnums gradually lose shocking power beyond 300 to 350 yards," although with larger calibers, "hydrostatic shock can occur on our medium game species at velocities as low as 2200fps." (Interestingly, the effects of hydrostatic shock can also decline if the velocity is over 3150 fps). 

    Like most every cartridge, the muzzle velocity of 10mm is well below even 2200 fps. Ballistics 101 has compiled the muzzle velocity of numerous commercial loads of 10mm. You will note that the typical muzzle velocities of the most widely distributed commercial ammunition is between 1000 and 1250 fps. Some of the more boutique manufacturers like Buffalo Bore, Cor Bon, Double Tap, etc., offer selections with muzzle velocities in excess of 1400 fps with one as high as 1600 fps. Even these are still well below the minimum necessary for producing hydrostatic shock. And, I would add, largely within the same velocity ranges as 9mm loadings.

    So we are left with the size of the wound channel, which as I noted is going to be a mix of the depth and width of the wound channel. Lucky Gunner has tested the 10mm in ballistic gelatin, and you can see the results here. Per FBI protocol, you can see that most of the defensive ammunition remained in the 12 to 18 inch penetration depth, with the bullets expansion in the range of .61 inches up to .81 inches. Compared with 9mm, the penetration for defensive ammunition is about the same, but as you expect from a smaller bullet, the expanded diameters are slightly smaller overall. Nevertheless, the newer and better quality defensive rounds such as HST and HST +P loads were up in the same range of expanded diameter. 

    So, even though 10mm is more "powerful" than 9mm, at least when looking at kinetic energy, the tests do not show that the 10mm is going appreciably more effective than the 9mm when using common police duty or civilian defensive loads. This is better illustrated by comparing the 9mm versus the .357 Sig which, notwithstanding its name, still uses the same .355 diameter bullets as the 9mm. The .357 Sig uses a larger case than the 9mm, but necked down to accept the .355 bullet. The purpose was to provide greater powder capacity and, therefore, greater energy--in other words, a more powerful round. But due to the constraints of the FBI protocol to avoid overpenetration, it doesn't actually appear to be more effective than 9mm in either tests or on the street. Chris Baker of Lucky Gunner actually discusses this in a video and article entitled ".357 Sig: What’s the Point of this Cartridge?" He relates, in part:

    Any time we test a new caliber, I like to do some background research on it. So a couple of weeks ago, I was reading an article published back in the Fall of 2000 by Dr. Gary Roberts, who is probably the most well-known wound ballistics researcher active today. He was sharing the results of a .357 Sig gelatin test he performed at the California Highway Patrol Academy range. This was printed in the Wound Ballistics Review, which was a scientific journal intended for hardcore ballistics nerds, so it tends to be pretty dry and technical most of the time. But at the end of this one article, Dr. Roberts breaks into editorial mode and he says,

    “Compared to a 9mm, the .357 Sig has a decreased magazine capacity, more recoil, as well as greater muzzle blast and flash, yet at best it offers no gain in bullet penetration and expansion characteristics. What is the point of this cartridge?”

    The 10mm offers slightly larger expansion--but still less than .1 inches--and the depth of the wound channel is pretty much the same. But if you were to compare the 10mm versus the .40 S&W, you would again be asking what is the point of the 10mm over the .40 S&W. 

    The reason the 10mm started seeing a resurgence was not because of defensive reasons, but in the realm of hunting and defense against wild predators. In that case, the bullet designs are not limited due to overpenetration. In fact, against a large animal such as a bear, the heavy hide, bones, and dense muscle mass calls for something with good penetration. This is where the 10mm really shines over lesser cartridges like the 9mm, .40 S&W, or even standard .45 ACP, because instead of all that extra energy being wasted, it will be converted to work: i.e., driving the bullet deeply into the body of the target. 

    Now, I will note that my discussion falls into the realm of "on paper" or "in theory" that Grant complains isn't a substitute for experience on the street, and he cites an officer that carries a 10mm as a backup to his duty 9mm, and uses the 10mm as his primary against drug crazed offenders. I can't speak to what the actual on the street statistics would show--I don't have that data. But I would note the following from Greg Ellifritz's study on the effectiveness of cartridges:

Something else to look at here is the question of how fast can the rounds be fired out of each gun. The .38 SPL probably has the slowest rate of fire (long double action revolver trigger pulls and stout recoil in small revolvers) and the fewest rounds fired to get an incapacitation (1.87). Conversely the 9mm can probably be fired fastest of the common calibers and it had the most rounds fired to get an incapacitation (2.45). The .40 (2.36) and the .45 (2.08) split the difference. It is my personal belief that there really isn't much difference between each of these calibers. It is only the fact that some guns can be fired faster than others that causes the perceived difference in stopping power. If a person takes an average of 5 seconds to stop after being hit, the defender who shoots a lighter recoiling gun can get more hits in that time period. It could be that fewer rounds would have stopped the attacker (given enough time) but the ability to fire more quickly resulted in more hits being put onto the attacker. It may not have anything to do with the stopping power of the round.

So, a 10mm may seem better to the cop on the street at stopping someone because it took less shots, but that in turn might only be because of the 10mm having such recoil that the officer was not able to shoot as fast with the 9mm giving the appearance of the 10mm being more effective because of the lower round count. 

    That does leave us with the question of what about the extra energy had by the 10mm? Is it just wasted, turning to heat rather than work? In soft tissue, I would say yes. But although I don't have the results of penetration tests against barriers, some of the testing I've seen in YouTube videos does seem to show that the 10mm is better for breaking down hard barriers such as cinder blocks and, perhaps, by extension, bone.