Why Four Layers of Denim Cloth?
There continues to be misunderstanding about testing JHP handgun bullet expansion using gelatin blocks covered with four layers of heavy denim cloth.
The four-layer heavy denim test was jointly developed by engineer Duncan MacPherson and California Highway Patrol to force manufacturers to design bullets that will expand more reliably when heavy clothing is encountered in actual shooting events. According to MacPherson:
Modern JHP handgun bullet designs perform very reliably in testing; expansion failures are rare. It seems likely that occasional expansion failures in service are inevitable, but the number of failures in [actual California Highway Patrol shooting incidents] appeared excessive to me even though they were a relatively small fraction of all shootings. The unavoidable conclusion seemed to be that these expansion failures were a result of the fact that the expansion of existing JHP bullet designs were not robust; in engineering terminology, lack of robustness simply means that small changes in conditions are likely to cause failure. Initially, this conclusion seemed surprising because “heavy clothing” stages have been common in handgun ammunition testing protocols ever since this approach was initiated by the FBI handgun ammunition test protocol defined in 1989, and the best modern JHP bullet designs have almost no failures either in these stages or against bare gelatin. A little more thought made this seem less surprising, because the “heavy clothing” stages in various tests seem to have been selected to represent specific clothing without any systematic investigation directed at evaluating what aspects of the cloth were critical.
A thoughtful investigation of the effects of soft barriers (e.g., clothing, as opposed to the hard barriers represented by building materials and automobile glass) seemed to me to be overdue. [California Highway Patrol Firearms Training Unit Lieutenant] Ed Fincel agreed with this assessment, and he, State of California Associate Procurement Engineer Nick Miloskovich and I set about implementing this investigation in the last quarter of 1996. This activity was very successful, and has led to a new ammunition test protocol [International Wound Ballistics Association (IWBA) Handgun Ammunition Specification]; ammunition satisfying the requirements of this test protocol has been developed [Winchester Ranger T] and is now commercially available in .40 S&W. This new ammunition has much more reliable expansion after penetrating soft barriers than any ammunition previously available in this caliber. Improved .45ACP and 9mm ammunition designs are in the final development stages.1
(MacPherson’s article presents 3-4 pages of additional, detailed information about how four layers of denim cloth was selected.)
The test protocol was established in 1998 by IWBA, which recently disbanded as an organization. It is superior to the FBI Heavy Clothing test event.
As described in IWBA Handgun Ammunition Specification Supplement, section 6.2:
Most expansion failures of JHP handgun bullets reported in actual shootings where hard barriers are not involved are probably due to factors that effectively plug up the hollow point cavity and reduce pressure in this area, although the dynamics model that occasionally leads to this result is not completely known in detail. This requirement in the IWBA Handgun Ammunition Specification is designed to force JHP bullet designs that expand much more reliably against soft barriers (hard barriers are discussed in more detail below). This requirement was selected after experimentation to provide a standardized, inexpensive, and precisely defined soft barrier that was a stressing but reasonable protocol for ammunition evaluation; it does not represent a simulation of specific clothing. The JHP bullet design features required to satisfy this requirement are well understood, and ammunition having these design features expands much more consistently and reliably against soft barriers than ammunition without these design features....
Therefore the four-layer heavy denim test is NOT intended to simulate any type of clothing; it is merely an engineering evaluation tool to assess the ability of JHP handgun bullets to resist plugging and expand robustly.
Properly prepared and calibrated 10% ordnance gelatin is the most accurate realistic soft tissue simulant currently available. It provides a reasonable indication of how a bullet can be expected to perform in soft tissues. All other barrier materials aside, clothing and bone are the primary reasons why a bullet recovered from a human body may not resemble one fired into a block of gelatin.
A well-designed bullet exhibits little difference in expansion and penetration between the bare gelatin test and four-layer heavy denim test. In actual shootings, performance usually falls between results exhibited in these two tests, unless bone is hit early in the penetration path. Thus bullets that expand reliably in four-layer denim testing perform well on the street.
Most modern, premium JHP handgun ammunition from U.S. manufacturers is designed to perform well against the IWBA four-layer heavy denim test.
Nowadays, unless the bullet hits bone or an intervening obstacle, or impacts the body at an extreme angle, it is more likely to perform in human soft tissues almost exactly as it performs in standard ordnance gelatin. More often than not, a bullet designed to perform well in the IWBA four-layer denim test that is recovered from a body looks like the same bullet fired into a block of gelatin. It did not used to be that way a decade ago, and criticism about the limitations of gelatin testing was valid indeed.
- MacPherson, Duncan: “Improved Handgun Ammunition.” Wound Ballistics Review, 3(3), 1998; pp. 12-21.
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