What Makes a Firearm Fully Automatic Under Federal Law

Under federal law, a firearm is fully automatic — a “machinegun” in the statute’s spelling — if it shoots, is designed to shoot, or can be readily restored to shoot more than one shot automatically by a single function of the trigger, without manual reloading. That definition lives in 26 U.S.C. § 5845(b) of the National Firearms Act, and it reaches further than most gun owners expect: it covers the frame or receiver of such a weapon, any part designed and intended to convert a weapon into one, and any combination of parts from which a machine gun can be assembled if those parts are in a person’s possession or control.¹

Three things about that language do most of the legal work. The weapon does not need to be currently functional; a broken machine gun that could be readily restored still counts. The definition is not limited to complete weapons; a single conversion part can be a machine gun by itself. And you can be charged for possessing the ingredients, not just a finished, firing weapon.

What “Single Function of the Trigger” Means

The dividing line is what happens each time you pull the trigger. A semi-automatic fires one round per trigger pull and requires the shooter to release and pull again for the next shot. A fully automatic keeps firing for as long as the trigger is held and ammunition remains. Even a burst-fire weapon that puts out two or three rounds per pull meets the federal definition, because more than one shot follows a single function of the trigger.

The Supreme Court sharpened this phrase in 2024 in Garland v. Cargill. The case involved bump stocks, which use a rifle’s recoil to let the shooter’s finger rapidly re-engage the trigger. The Court held that a semi-automatic rifle equipped with a bump stock is not a machine gun, because the trigger still has to reset between every shot; each shot results from a “separate and distinct function of the trigger,” even when the cycle happens very quickly.² The decision struck down the ATF’s 2018 rule classifying bump stocks as machine guns.

Cargill did not make bump stocks legal everywhere. More than a dozen states and the District of Columbia ban them under state law, independent of the federal classification.

Parts That Are Machine Guns By Themselves

Because the statute reaches conversion parts and combinations of parts, several small devices are themselves machine guns in the eyes of federal law, even sitting alone on a table.

A “Glock switch” is a small metal piece that attaches to the rear of a Glock pistol’s slide and prevents the trigger bar from resetting between shots, converting the pistol to fully automatic fire. The ATF classifies a Glock switch as a machine gun by itself, installed or not, because it is a part designed and intended to convert a weapon into a machine gun under § 5845(b).³ Possessing the switch alone, with no firearm attached, carries the same penalties as possessing an unregistered machine gun. The devices are widely available online for under $30, and many buyers do not realize they are committing a federal felony the moment the package arrives.

Drop-in auto sears and similar conversion devices sit in the same legal category. So does a collection of parts that could be assembled into a working machine gun if they are in one person’s possession or control.

Binary Triggers and Forced Reset Triggers

A binary trigger fires one round when you pull the trigger and a second when you release it. Because each direction of trigger movement is a separate function, these have generally avoided federal machine gun classification under ATF letter rulings. Letter rulings apply only to the specific device evaluated and can be revoked, so the legal footing is less stable than a court decision or statute.

Forced reset triggers use the firearm’s bolt carrier to mechanically push the trigger forward after each shot, resetting it faster than a human finger could. The ATF previously classified certain FRTs as machine guns, but a May 2025 settlement between the Department of Justice and device manufacturers resolved that dispute. Under the settlement, FRTs that require a separate trigger pull for each shot are not classified as machine guns.

The 1986 Ban on New Civilian Machine Guns

Even if a weapon meets every technical requirement, civilians cannot simply buy a new one. Under 18 U.S.C. § 922(o), it is unlawful for any person to transfer or possess a machine gun, with two exceptions: possession by or on behalf of a government entity, and lawful possession of a machine gun that was lawfully possessed before the statute took effect in May 1986.⁴

This provision, the Hughes Amendment to the Firearm Owners Protection Act, froze the civilian supply. Only machine guns already registered in the National Firearms Registration and Transfer Record before May 1986 can be lawfully owned by private citizens today. Every legally transferable machine gun in the country comes from that fixed, shrinking pool, and prices for registered transferables commonly run from $25,000 to over $40,000.

What Legal Ownership Requires

Buying one of those pre-1986 registered machine guns runs through the NFA transfer process. The transfer cannot happen until the ATF approves a written application that identifies both the buyer and the specific firearm, accompanied by the buyer’s fingerprints and photograph.⁵ The buyer cannot take possession until approval comes through, and the transfer carries a $200 federal tax paid by tax stamp.⁶ Recent legislation zeroed out the tax for suppressors, short-barreled rifles, and short-barreled shotguns, but machine guns still require the full $200. Median Form 4 processing times for individual applicants have been running under 30 days in early 2026.

State law is the second gate. Roughly 17 states and the District of Columbia prohibit civilian machine gun ownership regardless of federal registration. Most remaining states permit ownership only if the owner fully complies with federal NFA requirements, and about nine states impose no additional restrictions beyond federal law. Confirming your state’s position before wiring tens of thousands of dollars to a seller is worth the phone call.

Federal Penalties

Penalties are severe and can stack depending on which statute prosecutors charge. Under 18 U.S.C. § 924(a)(2), anyone who knowingly violates the machine gun possession or transfer ban faces up to 10 years in federal prison, a fine, or both.⁷ Separately, violating any provision of the National Firearms Act carries up to 10 years in prison and a fine of up to $10,000.⁸

These penalties apply equally to possessing an unregistered machine gun, possessing conversion parts such as a Glock switch or drop-in auto sear, and attempting to convert a semi-automatic firearm. Prosecutors do not need to prove the weapon was ever fired. Possession of the device or the parts is the crime, and the ATF has stated that enforcement against conversion devices is a priority, with federal prosecutions in this area rising markedly in recent years.³

1
Office of the Law Revision Counsel. 26 USC 5845
2
Supreme Court of the United States. Garland v. Cargill (06/14/2024)
3
Bureau of Alcohol, Tobacco, Firearms and Explosives. U.S. Attorney and ATF Release New Public Service Announcement Warning Against Possession of Machine Gun Conversion Devices
4
Office of the Law Revision Counsel. 18 USC 922
5
Office of the Law Revision Counsel. 26 USC 5812
6
Office of the Law Revision Counsel. 26 USC 5811
7
Office of the Law Revision Counsel. 18 USC 924
8
Office of the Law Revision Counsel. 26 USC 5871