This is my ghost gun. There are many like it.

[DJofSD]

http://www.wired.com/2015/06/i-made-an-untraceable-ar-15-ghost-gun/

This is my ghost gun. To quote the rifleman’s creed, there are many like it, but this one is mine. It’s called a “ghost gun”—a term popularized by gun control advocates but increasingly adopted by gun lovers too—because it’s an untraceable semiautomatic rifle with no serial number, existing beyond law enforcement’s knowledge and control. And if I feel a strangely personal connection to this lethal, libertarian weapon, it’s because I made it myself, in a back room of WIRED’s downtown San Francisco office on a cloudy afternoon.

The rest at the link.

[TJDave]

Aren't rifle barrels test fired?

[Clocker]

Aren't rifle barrels test fired?

The barrel generally does not have a serial number. Firearms like the AR-15 or most semi-auto pistols can be disassembled into a number of components. One of those components has a serial number, the only serial number on the weapon. As a practical matter, that component is the gun under law, and requires a license or background check to purchase. All other components are unregulated and replaceable.