On Wednesday, air marshals in Miami shot and killed a man who was pretending to carry a bomb off a plane. According to an official account, the marshals ordered the man to get on the ground and then opened fire when he reached into his backpack. A government spokesman says the marshals acted properly and that "this was a textbook scenario." If he really had been toting a bomb, could bullets have set it off?
It depends on the explosive. Some bomb materials are highly sensitive to impact; if you shoot a gun at a stick of dynamite, for example, there's a good chance you'll set it off. Others are less susceptible to gunfire. The military tries to make its explosives as durable as it can, since you don't want soldiers blowing up from the impact of a single bullet. A block of C4 plastic explosive can withstand a rifle shot without exploding. You can even set one on fire without too much worry.
That doesn't mean a bomb made from C4 (or another insensitive explosive like TNT) is impervious to gunshots. Such a bomb would have a detonator, which is far more vulnerable. The detonator serves as a mini-bomb that produces enough energy to blow up the main explosive. Here's how it works: A power source—usually a few batteries—provides an electrical charge that sets off a tiny explosion in one part of the detonator. This sets off another, somewhat bigger charge, which in turn ignites the payload of C4 or TNT. If a bullet were to strike the detonator, it could easily set off the more-volatile explosives stored inside.
You'd have to be an unbelievable shot to pull that off, though. In general, detonators are very thin—about the diameter of a pencil—and only a few inches long. If the man in Miami had been carrying a bomb, the chances of an air marshal accidentally shooting the detonator would be very small. If the bullet had hit the TNT, it might have passed right through. It's also possible that a stray bullet could disable the bomb. A bullet that happened to strike the batteries could jar them loose and cut off power to the detonator.
Although some terrorists use stolen, military-grade explosives, many rely on improvised bombs that tend to be far more sensitive. The shoe-bomber Richard Reid was trying to blow himself up with a very unstable mixture called triacetone triperoxide, which is brewed from acetone, hydrogen peroxide, and a strong acid. He had trouble lighting the fuse; a gunshot might have done the trick.
As for the man in Miami, we don't even know if the air marshals shot at him with conventional bullets. In the past, marshals have used special ammunition designed for airplane safety. One variety consists of little pouches of Kevlar filled with lead shot. While these could disable (and perhaps kill) a person, the distributed impact they produce would be less likely than a conventional bullet to blow up a bag of explosives. Bomb technicians shoot special types of ammunition at suspicious packages all the time: A slug from a water cannon or a burst of powder from a 12-gauge shotgun can smack the bomb in such a way that it breaks apart without going off.
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Explainer thanks Jack McGeorge of the Public Safety Group, Chris Ronay of the Institute of Makers of Explosives, and reader Drew Vance for asking the question.