AN AUTOMATIC M-16 rifle is missing from one of the Philadelphia Police Department's main gun-storage units and Commissioner Charles Ramsey has called in federal authorities to investigate.

"Obviously, this is very, very serious," Ramsey said. "And believe me, we will get to the bottom of it, one way or another, I guarantee that."

Ramsey said a regularly scheduled audit last week of 1,356 Colt M-16 rifles stored in a vault at the Philadelphia Police Academy revealed that one of the weapons was missing. In the previous audit in December, every firearm was accounted for, he said.

He offered several theories on the missing gun, including an inventory error or miscounting by an employee.

The worst-case scenario, Ramsey said, is that the gun may have been stolen by one of his officers. In that case, Ramsey said, he'd do everything possible to assure that the thief gets time in a federal prison - even though he wasn't quite sure what criminal statute would be applicable.

"I don't know what it is," he said, "but whatever the hell it is, we're gonna use it."

Only "a couple" of people have the combination code to the vault door, which is secured with a deadbolt and equipped with an alarm, Ramsey said. All of those people have been questioned, but will be questioned again when the Bureau of Alcohol, Tobacco, Firearms and Explosives conducts its investigation, he said.

Ramsey said he notified both the ATF and the FBI of the situation Monday.

He's also mandated an internal audit of every gun in every unit of the department.

The locks on the vault door will be changed and a surveillance camera will be installed outside the vault, Ramsey said, noting that it is "a day late and a dollar short."

The cache of Vietnam-era automatic M-16s was given by the military to police, who are converting the weapons to semiautomatic AR-15s, Ramsey said. Indications are that the missing weapon had not yet been converted, he said.

Ramsey said the department knows the missing weapon's serial number, so if someone tries to run it through a database, "it would pop," he said.

John McNesby, president of Fraternal Order of Police Lodge 5, said he was glad Ramsey decided to publicize the probe.

"We're not talking about a missing stapler here," McNesby said. "The sooner you get the word out, the better."

Ramsey said that the department has "never had anything like this happen to us before," and that he was not aware of another missing weapon in his five years with the department.

However, in 2011, following a Daily News investigation into allegations of impropriety in the department's Firearms Identification Unit, an audit by the U.S. Department of Justice conducted at Ramsey's request found that eight guns were missing from that unit's storage department.

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