Revising The Standards For Police Ammo Gelatin Tests
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 numerous shootings, we felt comfortable in making recommendations to departments that asked for them. For expanding bullets, the cartridge needed to produce a wound-cavity volume—expressed as energy transfer—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:
• Lead semiwadcutter (SWC) bullets had to hit the gelatin at a minimum of 900 fps to show any advantage over LRN bullets.
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