A man in Sacramento, California, has pleaded guilty to one count of unlawful manufacture of a firearm and one count of dealing firearms.

According to federal prosecutors, Daniel Crowninshield, known online as "Dr. Death," would sell AR-15 blanks, which customers would then pay for him to transform into fully machined lower receivers using a computer numerically controlled (CNC) mill. (In October 2014, Cody Wilson, of Austin, Texas, who has pioneered 3D-printed guns, began selling a CNC mill called "Ghost Gunner," designed to work specifically on the AR-15 lower.)

"In order to create the pretext that the individual in such a scenario was building his or her own firearm, the skilled machinist would often have the individual press a button or put his or her hands on a piece of machinery so that the individual could claim that the individual, rather than the machinist, made the firearm," the government claimed in its April 14 plea agreement.

As Ars has reported previously, the lower receiver part of a firearm is the crucial part that contains all of the gun's operating parts, including the trigger group and the magazine port. (Under American law, the lower is what's defined as the firearm itself.) The AR-15, the civilian version of the M-16 military assault rifle, is designed to be modular, meaning it can be assembled from different receivers, barrels, buttstocks, and other components, each with its own characteristics.

The sale of AR-15 blanks (sometimes known as "80 percent" lower) is not restricted. In fact, the metal piece is openly sold by various vendors, usually for under $100. Some of the vendors have received certification from the Bureau of Alcohol, Tobacco, and Firearms that these 80 percent lowers are not, in fact, firearms.

While anyone could mill their own AR-15 lower using either hand tools or a CNC, paying for someone else to do it constitutes a sale, which requires the seller hold a Federal Firearms License (FFL). Crowninshield did not have a FFL. Wilson, by contrast, does. However, Wilson does not take payment for milling nor does he transfer milled firearms.

“Great guy and great work”

As prosecutors describe it, on May 3, 2013, an undercover agent (UC) with the California Department of Justice arrived at Crowninshield’s shop, C&G Tool.

That agent brought with him two AR-15 blanks that he had purchased elsewhere. Crowninshield then:

…took one of the blanks and assembled a jig around the blank. Crowninshield then directed the UC to place the blank into a CNC machine, owned by Crowninshield at C&G Tool and used to mill out machine parts. Crowninshield then directed the UC to close the safety door on the CNC machine and to hit a specific button to start the machine. During this time, the UC observed another customer that appeared to be doing the same thing at another machine. The UC overheard Crowninshield and another employee discussing a customer purchasing blanks. These blanks were known colloquially as blanks because they purportedly constituted 80 percent of a completed firearm and could be milled out at machine shops into completed firearms. The UC inquired if Crowninshield sold such blanks. Crowninshield showed the UC an inventory of such blanks for sale. The UC purchased a blank for $260, a gray blank for $180, and an aluminum blank for $160, all from Crowninshield.

In total, the agent paid Crowninshield a total of $920 for those two blanks and for the service of milling the four lowers. Crowninshield never asked the agent to fill out any purchasing paperwork or complete a background check. He also didn't mark the lowers with any sort of serial number.

Previously, some gun enthusiasts would participate in "build parties," where they would gather at a CNC to finish off their product. However, the ATF began raiding these on-site build parties—but doing it yourself to achieve the same ends is still within the law.

Crowninshield advertised his services "on at least one online firearm enthusiast forum." In September 2013, he also advertised his new shop in San Jose, under the name "Dr. Death." One customer described Crowninshield and his services as a "great guy and great work. You will not be disappointed."

Crowninshield was criminally charged in June 2014 on six counts.

"The production and sale of assault rifles without serial numbers, background checks or waiting periods poses a serious danger to public safety," US Attorney Benjamin Wagner said in a statement in June 2014. "We will continue to investigate and prosecute those who seek to profit from such conduct."

Crowninshield is scheduled to be sentenced on June 30, 2016 in federal court in Sacramento. He could face up to 10 years in prison and a $250,000 fine.