Normally, buying merchandise through Amazon is an unremarkable experience. You click to purchase, then a tracking number appears in your e-mail, and, a few days after that, a driver pulls up out front. In most instances, you get what you paid for.

Not always, though.

On Tuesday night, Seth Horvitz, 38, a musician from San Francisco who moved to Washington a year ago, was alone in his Brookland apartment, getting ready for a dinner party, when he heard a knock at the door. By the time he opened it, a UPS man was leaving the building, on Eighth Street NE. To the right of the door, propped against a wall in the hallway, was a rectangular cardboard box about three feet long.

Horvitz, who composes and performs electronic music using computers, had purchased a 39-inch television for $324 through Amazon several days earlier, ordering it from a third-party seller. Having tracked the shipment online, Horvitz said, he knew that the big Westinghouse flat-screen was due to arrive early that evening.

But the box, which was addressed to him, seemed much too small.

“My thought was, maybe these are some accessories, and the TV will come in another box,” Horvitz said. He carried the parcel into his kitchen and used a key to tear away the packing tape. “When I first opened it and I saw the metal parts, I thought, ‘Oh, wow, maybe this is a TV stand.’

“Until I kind of put my hand on it,” he said. “Then I kind of jumped back, and I thought, ‘Oh, wait a second.’ ”

The merchandise — a tactical military-style SIG716 semiautomatic rifle, wrapped in heavy plastic and encased in Styrofoam, with an empty magazine for 7.62x51mm ammunition — came with an owner’s manual from the manufacturer, the German arms company Sig Sauer.

“SIG SAUER has taken the proven features of the SIG516 and applied them to a potent AR-based rifle chambered in 7.62x51mm,” the company explains, adding “the SIG716 is the rifle of choice when you require the power of a large caliber carbine.”

Horvitz, whose dinner guests would soon be arriving, required no such thing. In fact, before he reached into the box and felt the weapon’s pistol grip and trigger, he said, he had never held a gun. He hadn’t even seen one that wasn’t being toted by somebody in uniform.

“I definitely knew it was a mistake,” he said. “But I was confused as to how that kind of mix-up could happen. Especially given the recent events, the recent shootings. It surprised me to see how easy it would be for a gun to show up on someone’s doorstep — not just a gun, but an assault weapon.”

An Amazon spokesman did not return calls for comment. A UPS spokesman could not be reached.

Inspecting the box’s contents further, Horvitz said, he found an invoice showing that the rifle had been shipped by an online firearms broker,, for delivery to the Independence Gun Shop in Duncansville, Pa., a borough of 1,200 people about 90 miles east of Pittsburgh. The broker had been paid $1,589 for the weapon, according to the invoice.

A shipping label on the box bore the gun store’s name and address. A second label, with Horvitz’s name and address, also had been affixed to the box. Exactly who made the mistake remains unclear, as do the whereabouts of Horvitz’s TV set.

A few minutes after Horvitz opened the box, his wife arrived home, followed shortly by their eight guests. “None of us has any experience handling rifles,” Horvitz said, adding that he and his companions share a general aversion to firearms: “I personally would like to see all assault weapons banned,” he said.

The group enjoyed a potluck dinner with a North African and Mediterranean theme — the evening’s conversation enlivened by the surprise parcel. Horvitz, his wife and their friends stared at the gun, regarding it as a museum piece from an exotic culture.

“At first it seemed funny,” Horvitz said. “And then as it started to sink in more, it seemed like a more serious situation. We were totally shocked and confused. We sort of mulled over what to do. And we looked up quite a lot of information about the weapon.”

Finally, Horvitz phoned the D.C. police (and made another call, for good measure, bringing the matter to the attention of the Brady Campaign to Prevent Gun Violence). A while later, two patrol officers arrived at the apartment, asked a few questions, took a quick read of the invoice, then gathered up the rifle and drove away.

In tiny Duncansville, the gun shop owner, Jason Shirley, said that under federal law, someone buying a SIG716 on the Internet has to use a licensed firearms dealer as a conduit. Shirley said he was acting as a middle man for an occasional customer of his, a gun enthusiast he identified only as “a local surgeon.”

As for Horvitz, he just wants his TV.

“I’ve contacted Amazon,” he said, “and disputed the order.”