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Bullets and Baseballs

USC Medical Researcher and Engineer

November 12, 2004 —
A .45-caliber pistol bullet slams into soft body armor with the same impact as a 90 mile-per-hour fastball whacking a bare chest
This is one of the findings from an unusual study by a team of University of Southern California medical and engineering researchers that could lead to a better bulletproof vest, and possibly to super strong and "intelligent" armor.
"We studied how it would feel to get shot while wearing armor or bulletproof vests, " said Bart Kosko, a professor of electrical engineering in the USC Viterbi School of Engineering.  The armor experiments involved handguns and ordnance gelatin while the data analysis employed a novel mix of standard statistical methods and the more exotic neural-fuzzy rule-based techniques.
"This told us how much injury to expect from a handgun bullet hitting armor," Kosko said.  "We quantified the baseball comparison to help understand gunshots and armor. Handgun bullets are like baseballs. They do not knock people backward as in the movies. Instead they bruise soft tissue."
Kosko teamed with noted medical researcher W. French Anderson, director of the USC Keck School of Medicine Gene Therapy Laboratories, and Viterbi School Ph.D. student Ian Lee on the paper "Fuzzy Modeling of Gunshot Bruises in Soft Body Armor," published in the 23rd International Conference of the North American Fuzzy Information Processing Society held on 27-30 June 2004 (co-sponsored by the Institute of Electrical and Electronic Engineers).
Kosko has authored many books both technical and popular on fuzzy logic, a technique that uses shades of gray rather than simple yes/no answers and that allows humans to program computers with commonsense rules such as "If the air is cool then set the air conditioner to slow."
Anderson won international attention for being the first to cure a genetic defect in a human by transplanting a gene. Anderson also published a book, The Forensic Analysis of the April 11, 1986, FBI Firefight about a Miami shootout in which two FBI agents and their two assailants were killed, and five other police officers were wounded. The Federal Bureau of Investigation (FBI) has approved his report for release to law enforcement officials.
The research commenced when Anderson posed a simple question: What can science tell us about the bruising and other effects that result when someone gets shot wearing soft body armor?  There had been virtually no scientific work on the problem—yet, said Anderson, it is a question that many police officers and soldiers face every day. 
"The FBI had asked me to look into this, and I immediately thought of Bart Kosko.  Bart is an original thinker, and his ‘fuzzy logic’ sounded perfect for this task."  Anderson said.
The available evidence was scattered and incomplete.  The FBI published statistics showed that no handgun bullets killed armor-clad police officers by piercing.  The military inserts so-called trauma plates in a soldier's flack jacket to stop rifle bullets.
The National Institute of Safety and Technology (NIST) developed armor test procedures that used modeling clay to measure the backface deformation—the deformation in the armor's backing material after a gunshot.
But what does a 10-millimeter deformation in clay correspond to in a body? The researchers developed a trainable fuzzy system based on data that they obtained from numerous test firings of four different caliber bullets (.22, .38, .40, and .45) into ordnance gelatin-backed vests and other materials.  Their analysis was based on bruise patterns from actual shootings.  
The backface deformation depth and width on the armor estimated the trauma that would have been produced on the body beneath the armor. Timing established the average speed of each box of bullets. 
The simplest findings dealt with bullet momentum (mass times velocity) and bullet energy.  The energy of a speeding bullet grows with the square of its velocity.  But slower moving heavy bullets (like the .45 caliber) bruised more than high velocity lighter ones (like the .22 caliber). Impact depth correlated better with momentum than with kinetic energy.
To get a clearer and more intuitive understanding of the actual impact, the experiments backed the armor against Plumbers' Putty, measuring the impact, and then comparing the impact with that of baseballs hurled by a pitching machine.  While a .45 hits a bulletproof vest with the impact of a major league fastball in the ribs, a .22 is equivalent to being hit by a ball going 40 mph, the researchers found.
"This finding can help armor makers improve the tradeoffs involved in making thin extended-wear armor that minimizes gunshot bruises," Kosko said. "It can also help give police officers a better sense of what it feels like to be in the literal line of fire." 
"I often make presentations to law enforcement personnel about surviving deadly confrontations, but no one could explain in an understandable way what it feels like to be shot in the chest wearing soft body armor.  Finally we have something.  Being hit by a baseball is something we can relate to," explained Anderson.
The researchers believe new nanomaterials may also provide the protection of heavy bulky trauma plates in a lightweight flexible armor.  Other researchers have already shown that super tough nanotube threads can augment or replace the Kevlar fibers to strengthen armor vests.
A shear thickening fluid helps treated armor vests resist bullet penetration and reduce backface injuries. These and developments in smart materials and nanoscale devices promise super strong armor that can detect approaching bullets, report a soldier's vital signs and injuries, and then change armor nano-properties to react to threats. 
Such super strong armors that could process signals would be "intelligent" in the primitive but important sense that they would be able to make appropriate vest changes in response to new threat stimuli.  Ian Lee is pursuing such designs as part of his Ph.D. work in the new field of nano-signal processing.

The researchers used no public research funding of any kind for this study. Ammunition, rental firearms and facilities came from the Orange County Indoor Shooting Range, in Brea, California.  The researchers performed the baseball comparison trials in the batting cages at Home Run Park, in Anaheim, California.