The rifle’s extraordinary popularity can be traced to a number of factors, including the ease of its use, its embodiment of a certain military glamour, and the aggressive marketing of the gun industry.

The weapon was first built in the late 1950s by Eugene M. Stoner, a former Marine and the lead gun designer at the ArmaLite Division of the Fairchild Engine and Airplane Corporation. It was an unusual rifle for the era, made of lightweight plastics and aluminum instead of traditional materials like wood and metals. It also fired a .223-caliber bullet, which was smaller and faster than the typical ammunition at the time.

Partly for those reasons, the Pentagon, under Defense Secretary Robert S. McNamara, purchased the AR-15 in bulk in the 1960s and, after renaming it the M-16, turned it into the standard issue for American ground troops in Vietnam. Around the same time, a civilian version of the rifle, which unlike its military counterpart could not shoot automatically, also went on sale. Today, dozens of companies produce their own version of the weapon.

“Back in the day, people called it the ‘plastic rifle’ because it felt like a toy,” said Sam Andrews, the owner of Tier One Weapons Systems, a gun engineering company in Eureka, Mo. “But that’s evolved. Now people realize that light can be good.”

Because of its gas-operated system, Mr. Andrews said, the AR-15 has a fairly gentle recoil. The weapon is also fast and accurate, he added, able to fire, under capable hands, eight rounds in a second.