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Gun violence and gun control have been the subject of renewed debate since the February 2018 shooting at Marjory Stoneman Douglas High School in Parkland, Florida. In particular, those in favor of enhanced regulation of gun ownership have focused on the AR-15 semi-automatic rifle, which has been used in a number of high-profile mass shootings in recent years, including the massacre in Parkland. In June 2018, a dispute arose online over the physical impact and damage that can be caused by a bullet fired from an AR-15. ]Facebook user M.A. Rothman posted two photographs side by side: One featured a poster which claimed to show the size of a hole used by an AR-15, and the other showed what Rothman said was the true (smaller) dimensions of marks left by an AR-15 rifle after target practice: It's not clear what are the precise origins of the photograph on the left, or where it was taken. The poster has become the subject of ridicule and anger among some gun rights advocates online, prompting parodies and responses such as this one, which was shared more than 50,000 times on Facebook. In brief, both sides in this dispute are right in some ways and wrong in others, and each photograph offers an incomplete examination of the kind of damage and impact that can be caused by gun shots. At the heart of the issue is the question of what is meant by the hole made by an AR-15. There are basically three possibilities: the entrance wound, the exit wound, and the cavity caused by a bullet as travels through a person's body. There are no graphic images in this article, but we do at times get into descriptions of the damage caused by gun shots that some readers might find upsetting. The entrance wound The black hole shown in the poster is vastly too large to represent an AR-15 entrance wound. Jay Wachtel, a retired police sergeant and special agent in the Bureau of Alcohol, Tobacco, and Firearms, and a former criminal justice lecturer at California State University, summed it up in an e-mail: The entrance wound for any firearm projectile is essentially the bullet diameter. Typically, AR-15 rifles are loaded with .223 Remington cartridges, which are housed in bullets with a diameter of 5.7 millimetres. So a round fired correctly from a functioning AR-15 should create an entrance wound (or target practice marking) just under a quarter-inch in diameter. This can be seen in many AR-15 target practice videos and photographs available online, and is accurately reflected in the photograph posted by Rothman on Facebook. The internal cavity A bullet causes its most significant damage inside a person's body, destroying the blood vessels, tissue and bone in its immediate path (permanent cavitation), but also creating a kind of shockwave outwards into the tissue that surrounds this path. This is known as temporary cavitation, and when a bullet travels at higher velocity, the damage caused by this type of cavitation becomes exponentially greater, because the surrounding tissue struggles to absorb the kinetic energy given off by a bullet traveling at a higher velocity. The design of bullets also has a bearing on temporary cavitation (for example, hollow point rounds), but high velocity is a major reason why rifles such as the AR-15 can wreak such catastrophic damage to internal organs, and cause fatal bleeding. This mechanism is explained by the ammunition manufacturer Hornady: Heather Sher, a Fort Lauderdale-based radiologist who examined CT scans from some of the victims of the February 2018 Parkland school shooting, wrote about the effects of AR-15 rounds for the Atlantic: The hole illustrated in the poster could plausibly represent the diameter of the temporary cavitation left behind by an AR-15 rifle round traveling through the human body. That is to say, the tract of potentially catastrophic damage done to blood vessels and tissue could be as tall and wide as the hole shown in the poster. The exit wound The black hole illustrated on the poster is unlikely to be representative of the size of an AR-15 exit wound — but it is possible. While entrance wounds typically have the same dimensions as the bullet that causes them, the size and shape of exit wounds can vary more. Dr. Sydney Vail is a trauma surgeon with the Maricopa Integrated Health System in Phoenix, Arizona, and is an expert in treating gunshot trauma. He explained to us some of the factors that determine the dimensions of exit wounds: Dr. Judy Melinek, a forensic pathologist from San Francisco, California, explained to us the circumstances in which an AR-15 might leave behind an unusually large exit wound. By email, she told us: In her assessment, an exit wound the size of the black hole shown in the poster could potentially be caused by an AR-15 round, but it is not likely, and certainly not typical. Melinek told us that in her experience, AR-15 rounds can completely fragment in the body and may not exit at all. Michael Knox is a Florida-based forensic consultant who conducts ballistics testing and crime scene reconstructions, and is regularly called upon to give expert testimony in criminal trials. His assessment of the black hole on the poster was blunt. By e-mail, he told us: The exit hole from an AR-15 round would not be that big. Conclusion The photographs shown in the viral Facebook post are a useful conversation-starter, but comparing them doesn't serve readers well as an explanation of the physical impact of gun shots in general, or high-velocity AR-15 rounds in particular. Based on the assessments shared with us by the experts we consulted — a forensic pathologist, a trauma surgeon, a former federal agent and criminal justice lecturer, and a forensic consultant and ballistics expert — we can say that the entrance wound from an AR-15 round could not be anywhere near that large; however, the tract of catastrophic damage torn through a person's internal blood vessels, tissue, and bone by a high-velocity AR-15 round could plausibly have around the same diameter as the black hole in the poster. Finally, it is unlikely that an AR-15 exit wound would have the same dimensions as the black hole shown in the poster, but it is possible. The bullet would have to travel through the body in such a way that it fragmented bone and tissue to the extent that all that matter was forced out of the body along with the bullet itself, creating an exceptionally large exit wound.
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