The article stressed expansion and indicated penetration was not the problem. While exact penetration depth was not stated, the article seems to indicate penetration met or exceeded the FBI requirement of twelve inches. Here’s an excerpt from the article, beginning with the last paragraph on page 38.
[quote, bold & underline added]
The FBI has established that a handgun bullet must consistently penetrate a minimum of twelve inches of tissue for reliable penetration of vital organs in the human body. The FBI Wound Ballistic Testing Protocol, also known as the FBI Ammo Test Protocol, is a series of tests designed to assess a bullet’s ability to inflict effective wounds after encountering various barrier materials (automotive windshield glass, automotive grade sheet metal, drywall and plywood) commonly encountered in law enforcement shooting scenarios.
Unfortunately, many of the bullets that perform well in the FBI tests fail to adequately expand when heavy clothing is encountered. According to the IWBA, this is a performance deficiency that’s common across the ammo spectrum. The IWBA Handgun Ammo Specification was developed to address this problem.
The IWBA Handgun Ammo Specification is simple in nature. It consists of two tests: 1) bare gelatin, and 2) gelatin covered by four layers of sixteen ounce denim cloth. The denim cloth isn’t intended to represent any particular item of clothing that may be worn by a criminal adversary. Rather, it’s intended to provide a reasonable test of a bullet’s ability to expand after passing through cloth. Federal HST was designed to perform well in both the FBI and IWBA tests.
The expansion and penetration performance of HST bullets exceed that of most other bullets. According to Federal, previous and current generations of hollow point rounds were and are designed to expand to 1.5 times the original bullet diameter in bare gelatin. In contrast, Federal’s HST rounds are designed to expand well in excess of twice the original diameter in bare gelatin. HST bullets typically have a larger expanded diameter than other designs for the same penetration depth. HST has a unique upset profile and creates a large wound channel.
Although penetration depths are not defined, it seems to me the HST meets or exceeds the FBI requirement of at least 12 inches.
While that's fine for a side shot (e.g. arm/lung/heart/lung), 12 inches (or more) penetration from the front of most large men would create both an entry and an exit wound--a good thing to stop a bad guy, a frightening thing to anyone on the other side. "Backstop" remains an important concept.