Aurora Gunman’s Arsenal: Shotgun, Semiautomatic Rifle and, at the End, a Pistol

The three types of weapons used by the man accused of killing 12 people in a Colorado movie theater — a semiautomatic variation of the military’s M-16 rifle, a pump-action 12-gauge shotgun and at least one .40-caliber semiautomatic pistol — are among the most popular guns available in the multibillion-dollar American firearms market.

.. It appears, the police say, that James E. Holmes, the man accused in the Aurora shootings, used all three types of weapons inside the theater as well, first firing the shotgun, then using the semiautomatic rifle until its 100-round barrel magazine jammed, and finishing off with a pistol. (A second .40-caliber pistol was also found at the scene, though it was unclear whether it had been used in the theater.)

.. Many other details about the rampage also remain unclear, like whether the gunman used soft-point or full-metal-jacket ammunition, or whether any of the firearms had been modified with scopes or night-vision devices.

.. the M-16, the signature weapon of the Vietnam War

.. If anything, the experts said, a shotgun in that situation might have been the most lethal, since every shell can spray a half-dozen or more pellets, each capable of killing or maiming a person. Twelve-gauge shotguns often fire five shells, and sometimes more

.. “Shotguns are a very good antipersonnel weapon at close range,” said John C. Cerar, the former commander of the firearms and tactics section for the New York Police Department.

.. AR-15, marketing more than a dozen models that range in price from about $700 to $2,000.

.. it is easy to handle and can be modified in numerous ways. Some soldiers call it “a Barbie doll for men” because it has a wide range of accessories and replacement parts, including different styles of barrels, stocks, magazines and scopes.

.. The M&P15 also comes in a variety of models that fire different sizes of ammunition, from .22-caliber to .30-caliber rounds.

The rifle used in Aurora fired .223-caliber ammunition, law enforcement officials said.

.. Those rounds — similar to the ammunition used in American M-16 and M-4 rifles — are smaller than the rounds fired by Afghan insurgents wielding Kalashnikov rifles, but pack far more power than .22-caliber rounds, even though they are only a hair’s-width larger in circumference.

.. Law enforcement officials said the 12-gauge shotgun used by Mr. Holmes was a Remington 870. The gun, which can be purchased for around $400, requires the user to pump a handle underneath the barrel to chamber new cartridges after each shot.

.. The handgun was a Glock .40-caliber semiautomatic pistol, the officials said. The weapon is similar to the 9-millimeter handgun made famous in gangster films and rap music. The 9-millimeter has also been adopted by many police forces that had used .38-caliber revolvers and felt that they were being outgunned by criminals.

.. The .40-caliber Glock, versions of which sell for about $400, has become increasingly popular partly because its larger round makes it potentially more deadly than a 9-millimeter

.. Yet it has less recoil and is thus easier to handle than a .45-caliber handgun.

.. The pistols typically come with magazines capable of holding 13 to 17 cartridges.

.. Pistols are less accurate than rifles at longer distances and are widely considered harder to use because they cannot be braced against the shoulder. But in close quarters, they are plenty lethal.