In the 1920s and ’30s, America faced a wave of violence as gangsters like Al Capone and “Baby Face” Nelson used machine guns to shoot nonstop at cops or rivals. In 1934, the same year Baby Face was killed in a FBI shootout, Congress passed the National Firearms Act, making it illegal for Americans to manufacture fully automatic weapons for personal use. In 1986, it became illegal for civilians to own newly made machine guns. But with just $299, you can modify your semi-automatic AK-47 and probably fool your neighbors:

Slide Fire is a company that sells gun stocks that you can use with an AK-47 or an AR-15. These attachments enable accurate “controlled rapid firing,” according to the company’s website, meaning “you can shoot one round, 2 rounds…15 rounds or a full magazine,” as Jeremiah Cottle, the US Air Force vet who invented the product, told Guns America last year.

Gun enthusiasts, who have posted videos on YouTube of the Slide Fire in use, seem to love the product. “It’s just like an M-16!” the shooter in the YouTube video above exclaims. “You can shoot it accurately…or if you want to have fun, you can just spray the shit out of everything.” Survivalists also love the product. “If the gun-grabbers and Brady camp gets hold of this, it’s game over,” wrote a user on a survivalist message board, referring to the Brady Campaign to Stop Gun Violence.

A gun modified with Slide Fire “fires as a machine gun would,” explains David Coulson, a spokesman for the Bureau of Alcohol, Tobacco, Firearms, and Explosives, the federal government’s gun regulator—but, he adds, that doesn’t make it a machine gun.

That’s because, despite enabling rapid fire that mimics a fully automatic weapon, Slide Fire doesn’t appear to violate the production ban in the National Firearms Act. The law only regulates weapons that are designed to shoot “automatically more than one shot, without manual reloading.” The way the Slide Fire works, as Cottle explained to Guns America, makes it easier for semi-automatic gun owners to do what they’ve been doing anyway: “bump firing,” which is where you simulate automatic firing by rocking the gun against the trigger finger. (This practice can also lead to highly inaccurate shooting.)

The Slide Fire helps shooters increase their accuracy and number of rounds—without actually firing automatically. “You actively fire every round, and if you stop pushing forward or you take your finger off the trigger the gun stops firing. It just helps you fire the gun in semiautomatic very fast,” Cottle told the magazine.

A letter from the ATF on Slide Fire’s website certifies the part’s legality for exactly that reason. According to the letter, Slide Fire told ATF that the stock “is intended to assist persons whose hands have limited mobility.” So, basically, if you are missing a hand and need to fire a gun like Capone, the Slide Fire is for you.

Coulson, the ATF spokesman, confirmed the stock does not violate any national laws, and Stephen Halbrook, an attorney who has represented gun companies and the NRA, says “if it was even borderline illegal, ATF would have told them, no, you can’t market it. I’ve met the guys there and they’re very strict.” In 2006, the ATF revoked its approval of a strikingly similar piece, the Akins Accelerator. But the Akins Accelerator, unlike the Slide Fire, included a recoil spring in its design. (It has since been successfully relaunched without the spring.)

Although the federal government is okay with the Slide Fire stock, states have a role in gun regulation, too. According to company’s website, it “has not been notified by any individual state that our products conflict with any state laws.”

But even if it’s legal, it’s still easy to confuse a Slide Fire-modified weapon with a fully automatic weapon. Because it sounds so similar to banned kits that convert semi-automatic guns to fully automatic, says James Wright, a gun policy expert who teaches sociology at University of Central Florida, “You would think the National Firearms Act would cover something like that. But if it’s available online, I guess it’s legal.”