The AR-15-style rifle has become a common weapon for consumers, but it’s also become a weapon of choice for mass shooters. Gunmen have used them to kill dozens in horrific rampages across the country. Some of the factors that make the AR-15 popular with consumers may also help explain why mass murderers have opted to use them too. They’re capable of firing rounds that can go straight through a human body and penetrate a wall behind it. They’re lightweight and easy to use. Since the muzzle doesn’t rise up much, it doesn’t take a lot of effort to bring it back down, acquire a proper sight line and to fire again. A weapon that’s already deadly can potentially be made more dangerous by adding things like foregrips, scopes and red dot sights. The guns are expensive. But they’re within reach. They cost between $500 and $900. The AR-15 model is basically a consumer version of military-grade assault rifles such as the M-16 or the M-4 carbine. They look almost identical. The big difference: The AR-15 and its variants are semiautomatic; the M-16 and M-4 can be switched to automatic or burst. AR-15-style rifles were prohibited under the assault weapons ban from 1994 to 2004. But since that ended, sales have surged. These weapons have been branded under the term “modern sporting rifle,” which gives a weapon designed for war a consumer-friendly label. The gun is also marketed for home defense. But although they’re clearly strong enough to stop an intruder, critics say AR-15-type rifles could also cause unintended damage or even death in untrained hands. One thing is clear: No matter who’s using it, it can be an extremely lethal weapon.