Revising The Standards For Police Ammo Gelatin Tests
January 04, 2011
Last month, I left you with a shocking revelation: In 1974, the Dallas Crime Lab where I worked recommended the right .38 Special load to the Dallas Police for the wrong reason. How did we find this out?
In the initial stage of testing police ammo (1972 to 1973), we recommended that Dallas replace its lead roundnose (LRN) ammo with a then-new Winchester load that pushed a 158-grain lead hollowpoint (LHP) bullet at what today would be +P velocities. We based our decision largely on a military method of measuring temporary wound-cavity volumes in ordnance gelatin that we modified for handgun ammunition. The program's success led to the lab obtaining better chronographs and formulating a plan to evaluate as much commercial handgun ammo as we could.
With the chronograph upgrades, we could make simultaneous readings of a bullet's entrance and exit velocities by placing independently wired chronograph screens on either side of the gelatin block. The new setup also let us gather more data, and we crunched them in a number of ways.
As with the .38 Spl., we tested other police cartridges by first evaluating the original load for each. For the .357 Magnum, it was the old 158-grain lead semiwadcutter; for the .45 ACP, we used the 230-grain FMJ roundnose. We used test firearms that were common to police carry, not unrealistically long test barrels. You can see these "baseline" wound volume numbers in the accompanying chart plus a range of newer loads we tested. Remember that the 10mm and .40 S&W cartridges didn't exist then. Don't be surprised that we included the .41 and .44 Magnums. A fair number of Texas cops packed the big revolvers in those days.
As we moved to newer ammo types, we continued to review officer-involved shootings to sharpen our ability to predict the success or failure potential of a load. Some of the newer .357 Mag. HP loads posted very high numbers in our tests, with temporary wound cavities often six times larger than the baseline .38 Spl. LRN could manage. And this, good reader, is where reality clashed with theory as we discovered that the military wounding standards we borrowed were too narrow for expanding handgun bullets.
A Typical "Failure" Scenario
A single incident will suffice to illustrate our problem; many others had similar elements:
Two patrolmen asked a man for identification. Instead, the suspect produced a .25 ACP pistol and dropped one officer with a bullet to the kneecap. The second officer quickly emptied his .357 Mag. revolver at close range, only to watch the suspect stroll away as if nothing had happened, still holding the little pistol at his side.
The uninjured officer managed to fumble three or four fresh cartridges into his revolver and ran after the suspect. He yelled to turn the man and was taking up the trigger slack when the suspect's eyes rolled back in his head and he fell. He had been hit hard in the first volley, but the effects were delayed. The injured officer recovered uneventfully, but the shooter remained comatose and died about a week later.
The medical findings were eye-poppers. In the initial flurry of .357 Mag. bullets, one fired parallel to the plane of the shoulders struck the suspect in the left armpit and traveled across the left lung and lodged lightly in the heart muscle. The officer had purchased a 110-grain JHP load with very fragile bullet construction; the jacket appeared to have been designed for .38 Spl. velocities. The wound-cavity volume the load produced in gelatin was about 51â„2 times that of the baseline .38 Spl. and very high among wound volume producers, yet it failed to stop a dangerous man.
BASELINE WOUND VOLUMES
|CARTRIDGE ||BARREL LENGTH (in.) ||BULLET WEIGHT (grs.) ||BULLET STYLE ||BASELINE WOUND VOLUME* (ft.-lbs.) ||WOUND VOLUME RANGE (Newer Loads) |
|.25 ACP ||2 ||50 ||FMJ RN ||51 ||(not tested) |
|.380 ACP ||3.25 ||95 ||FMJ RN ||74 ||133 to 170 |
|.38 Special ||2 ||158 ||Lead RN ||62 ||137 to 214 |
|.38 Special ||4 ||158 ||Lead RN ||77 ||136 to 392 |
|9 mm Luger ||4 ||115 ||FMJ RN ||107 ||176 to 377 |
|.38 Super ||5 ||130 ||FMJ RN ||138 ||376 to 393 |
|.357 Magnum ||4 ||158 ||Lead SWC ||166 ||212 to 502 |
|.41 Magnum ||4 ||210 ||Lead SWC ||188 ||498 to 618 |
|.44 Special ||4 ||246 ||Lead RN ||69 ||194 |
|.44 Magnum ||4 ||240 ||Lead SWC ||636 ||538 to 639 |
|.45 ACP ||5 ||230 ||FMJ RN ||114 ||170 to 353 |
|.45 Colt ||4 ||250 ||Lead RN ||159 ||165 to 386 |
|* Temporary cavity is expressed as ft-lbs of energy transferred to ordnance gelatin. Newer loads were tested from 1974 to 1986. |
The suspect was part of the problem. In addition to being heavily muscled, he carried a load of fat around his upper chest and armpit areas. The fragile bullet had to traverse much more tissue than in a face-on shot to an average person. It also shed nearly half its weight, reducing the energy carried to the vital organs. The wound evidence indicated that the bullet velocity was very low as it penetrated the left lung. In the 30 to 45 seconds between taking a fatal hit and actually going to ground, he could have done a lot of damage with his little .25 ACP.
Revising The Standards
We needed to find out what was missing from the picture. Officer-involved shootings gave excellent data. We had the gun and ammo evidence in hand, and we usually interviewed the officers. We could get very accurate descriptions of the wound tracks from medical examiners and trauma surgeons. And then there were the gelatin blocks.
Being rather amazed at the diverse shapes of wound tracks in the nearly transparent gelatin, we had sketched many. Like tissue, gelatin springs back to original size after being shot, but its transparency reveals shear lines that closely define the position of the temporary cavity. Not immediately seeing a numerical way to evaluate cavity position, we started comparing street results with the cavity shapes produced by the ammo types. The cloud began to lift.
Ammunition with numerically large wound cavities yet posting poor street results often featured lightweight or lightly constructed bullets at high velocity. This included many .357 Mag. and 9mm Luger loads. Their cavities tended to reach maximum diameter in the first 2 inches of the 51â„2-inch gelatin block. The remainder of the cavity was basically bullet diameter, and some loads failed to completely pass through the block.
Reviewing cartridges that produced good street results showed the cavity shapes were much different. These produced cavities that were distinctly shaped like a football, with maximum diameters positioned around 3 to 4 inches into the blocks. We had fallen upon a key factor. Although wound-cavity volume as determined by military methods was important, the distribution of that cavity was equally if not more important. We had numerous shooting results where a cartridge producing a modest cavity volume of proper distributionâ€”like the .38 Spl. lead HP load we recommendedâ€”was more effective in the real world than cartridges making a much larger cavity that was too shallow.
I finally found factors in our data that helped us numerically define cavity position. We calculated the transfer ratio: the bullet energy transferred to the block divided by the striking energy. If a cartridge had 400 ft-lbs of kinetic energy when it reached the block and 100 ft-lbs at exit, then it delivered 300 ft-lbs to the block. Dividing 300 by 400 gives a ratio of 0.75.
By working through shooting records where we had good evidence of the outcomes, we found the most successful cartridges had transfer ratios between 0.70 and 0.90. Ratios falling much over 0.95 produced too many poor outcomes, especially when the suspect presented a side shot or was of heavy build, both fairly common scenarios. Light bullets that shed their energy too close to the entrance site had trouble reaching vital areas. The .357 Mag. ammo that failed to quickly incapacitate in the shooting above had a transfer ratio of 0.97.
This showed the need to test everything and assume nothing. Just because a .357 Mag. bullet was fast and light did not necessarily mean it was destined for failure. The 110-grain .357 Mag. bullet that failed was fragile and made a shallow cavity; another brand with a nearly identical wound volume but a tougher bullet construction had near-perfect distribution with a transfer ratio of 0.88. Police administrators and officers alike began to realize that you couldn't predict success from ammo catalogs, salesmen, or the physical appearance of a cartridge.
It was fortunate that we discovered the role of the transfer factor as early as we did; at that time we had made no recommendations other than the .38 Spl. Once we had this backed by the results of num
erous shootings, we felt comfortable in making recommendations to departments that asked for them. For expanding bullets, the cartridge needed to produce a wound-cavity volumeâ€”expressed as energy transferâ€”of at least 200 ft-lbs and have a transfer ratio of 0.70 to 0.90. Higher wound volumes were good; higher transfer ratios were bad.
Other Stuff We Learned
We accumulated a wealth of extra information that revealed some basic truths about handgun bullet behavior. Here are some examples:
â€¢ Lead semiwadcutter (SWC) bullets had to hit the gelatin at a minimum of 900 fps to show any advantage over LRN bullets.
â€¢ Jacketed softpoint bulletsâ€”popular then as being noncontroversialâ€”had to exceed 1,300 fps to start any expansion, although they were capable of reasonable if not stellar outcomes if loaded over 1,100 fps.
â€¢ When choosing .357 Mag. ammo, we had to consider barrel length. With the slow-burning propellants used, barrels longer than 4 inches could produce velocities exceeding the construction limits of some hollowpoints, producing shallow cavities not seen when we used shorter barrels. We ultimately recommended bullets in the 140- to 158-grain range for longer barrels.
â€¢ Original .44 Mag. lead SWC loads started to expand with 4-inch barrels and produced a wound cavity as large as newer .44 Mag. HP loads but with unacceptably high exit velocities.
â€¢ The .41 Mag. "police" loadâ€”a 210-grain lead SWC at 960 fpsâ€”struggled to make the 200 ft-lbs minimum we set and nearly always produced a hazardous over-penetration situation. In spite of this, it proved to work well in the "real world" as long as no one was standing behind the bullet's intended recipient.
We also learned how in-tissue bullet stability affected wound-cavity volumes:
â€¢ Standard-velocity lead RN .38 Spl. bullets (158 grains) began to tip at about 31â„2 inches into the gelatin, as shown by a flare line extending from one side of the wound track. This slightly increased the cavity volume.
â€¢ High-velocity lead RN .38 Spl. bullets stayed nose-on through the entire block, as did all lead SWC
.38 Spl. ammo fired at similar velocities.
â€¢ The .44 Spl. 246-grain LRN at 660 fps from a 4-inch revolver was so stable in gelatin that it produced wound volumes that were slightly less than the unstable .38 Spl. LRN.
â€¢ Rifling twist rate can affect in-tissue stability. We fired factory
.45 Colt ammo from two revolver makes with different twist rates and barrel lengths. Although velocities and muzzle energies from the shorter barrel were less, the slower twist let the bullet start to tumble in the gelatin and increased the wound-cavity volume by a modest but statistically significant amount. See the accompanying chart on the next page.
Sometime in the early 1980s, our lab's caseload became so oppressive that we didn't have time to continue the ammo-test program on a regular basis. Any testing was at night or on weekends, slowing the program. After I left the lab in 1987, our section chief decided that the new FBI ammo test protocol was becoming the de facto standard, and all testing in Dallas ceased.
The FBI tests focused on the size of the permanent wound cavity and total depth of penetration in long gelatin blocks; ours determined the size of the temporary cavity and how it was distributed in short blocks. Were these two systems mutually exclusive? Not at all. Years later, I obtained the early FBI test results and compared them to the Dallas results where the same loads were tested. Most of the loads that the FBI found to have good performance also tested well on our system.
When I traveled to present our program results to various police academies around the Southwest, some recruit invariably asked what I chose for personal protection after testing hundreds of loads. The answer was consistent: a custom Colt Government Model 1911 with Speer's old 200-grain "Flying Ashcan" hollowpoint. I never had to use it in anger but was always confident that a solid evaluation program supported my decision should a situation "go bad."
I started this column by saying we recommended the right cartridge for the wrong reason. Our recommendation of the specific .38 Spl. LHP loading was based solely on cavity volume. As we later discovered through more detailed testing, the cartridge also produced optimum wound cavity distribution. The Dallas PD stuck with the LHP until issuing 9mm Luger sidearms in about 1989.
From all of this, I learned two unavoidable facts: First, there's no such thing as a "magic bullet." Even the mightiest gun/cartridge combos had the rare failure to stop. Even more basic was the second fact: A center hit with a .38 roundnose always outperforms a miss with a .44 Mag.