Aguilar arrived at Millen's shop with little experience with guns, so he gave the young man a quick tutorial on how to fire and care for the weapon — an interaction that haunts him in retrospect.

Millen placed the gun in Aguilar's hand and showed him where the safety was. He told him not to point the gun at anything he didn't intend to shoot. Millen warned Aguilar against carelessly putting his finger on the trigger.

Millen didn't demonstrate how to load shells into the gun but showed him where they go. He urged him to go to a shooting range so he could see if the gun was really what he wanted.

Although the gun can chamber six shells at a time, Millen said he recommended that Aguilar load one shell every time he was going to fire so the novice didn't accidentally shoot again when the gun kicked back.

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Lean forward, Millen said he told Aguilar, because the Mossberg had a "good amount of kick."

Millen showed him how the gun's 18.5-inch barrel comes off easily for cleaning or storage. It allows the gun to be carried in two pieces instead of one 3-foot unit.

He asked Aguilar to fill out a mandatory Bureau of Alcohol, Tobacco, Firearms and Explosives application, which asks prospective gun buyers pages of questions, including whether have been drug users, fugitives or ruled "mentally defective" by a judge.

Millen made copies of Aguilar's Maryland learner's driving permit and his state change-of-address form. Then he plugged in the teen's Social Security number and other information into an FBI National Criminal Instant Background Check system. Within a couple of minutes, he said, the check came back with a response: "Proceed."

From walk-in to sale, the entire transaction took about 30 minutes. Aguilar bought a box of Hornady-brand buckshot and a box of Federal-brand bird shot along with the gun. A few weeks later, around Christmas, Aguilar came back into the shop with a friend and told Millen that he had taken his advice and tried out the gun at a shooting range. He told the gunshop owner the shotgun had given him quite a kick, as predicted.

Millen joked with Aguilar and said he called him a "lightweight," which drew a chuckle.

"He just said he was going to pick up another box of the birdshot," Millen said.

He did and left.

"I just feel horrible [about] two innocent people, that their lives were taken," Millen said. "Two kids."

While he is a gun-rights advocate, he said he believes in limited regulations to prevent gun violence. In this case, he likened the transaction to a car dealer selling a vehicle to somebody who gets drunk and drives.

Millen said he plans to engage his customers more, inviting them to gun ranges for safety lessons and to get to know them better. He's done that with some patrons, and wishes he did so with Aguilar.

"They're deadly," he said. "They are when they're in the wrong hands. But they don't have to be."

Baltimore Sun reporters Liz Bowie, Erin Cox, Ian Duncan and Nayana Davis contributed to this article.