Legal analysis

President Donald Trump has delegated to Attorney General Jeff Sessions the task of preparing regulations to ban bump stocks "that turn legal weapons into machine guns."

As the attorney general, Sessions has the responsibility to enforce the Gun Control Act of 1968 (GCA), and the National Firearms Act of 1934 (NFA), a task which is in turn delegated to the Bureau of Alcohol, Tobacco and Firearms (ATF). Since 2010, however, the ATF has taken the position that bump stocks are "not regulated as a firearm under Gun Control Act or the National Firearms Act." The ATF does not have the authority to regulate firearm parts and accessories.

It seems the president has asked his agencies to issue regulations banning a thing that is not considered regulable by the same agencies.

The ultimate issue is whether bump stocks turn legal weapons into (mostly) illegal machine guns.

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The NFA defines "machine gun" as any weapon that "shoots, is designed to shoot, or can be readily restored to shoot automatically more than one shot, without manual reloading, by a single function of the trigger."

The GCA regulates the transfer and possession of machine guns, and makes it unlawful for any person to possess a machine gun unless it was lawfully possessed before a certain date or is under the authority of a federal or state government.

Bump firing uses the recoil of a legal semi-automatic firearm to fire like a fully automatic weapon. Bump fire can be accomplished without any special stock or equipment, by loosely holding the rifle in a way that allows it to bounce against the trigger finger. A thumb threaded through a belt loop while holding the rifle can achieve bump fire, albeit with little control or accuracy. The firearm itself is not mechanically altered — the bump fire technique just uses a fundamental, legal principle of semi-automatic fire, and physics.

The bump fire stock attaches to rear of an AR-15 or similar rifle. The stock itself has no automatically functioning mechanical parts or springs; it performs no automatic mechanical function. The shooter does not fire normally; the shooter applies forward pressure with the nonshooting hand, and rearward pressure with the trigger hand. This is why the ATF originally concluded that the bump stock is just a firearm part, which makes it not subject to regulations, at least under the GCA or the NFA.

So, then, Trump has arguably ordered regulations banning something that may not be subject to regulations.

Even if the president could ban the bump-fire stocks, that alone would not end bump firing. A legal semi-automatic weapon like an AR-15 could still be bump-fired with a sturdy pair of jeans.

The ATF could separately set a limit on the firing rate of semi-automatic weapons, but that would be hard to enforce. Would law enforcement have probable cause if they heard distant gunfire and estimated the cyclic rate in excess of a set number of rounds per second? In another situation, a particularly talented or fast shooter could commit a felony by simply pulling the trigger too quickly.

Gun owners would argue that a standard rate of fire might cast an overly wide net that would criminalize constitutional use. And realistically, a standard rate of fire wouldn't stop folks from taking their legal rifle into the woods to bump fire it against their jeans.

After the Las Vegas shooting, the Justice Department indicated the ATF will reconsider the definition of "machine gun" to clarify whether bump fire stocks fall within the definition. The implied pressure is clear: Find a way to conclude that bump stocks constitute illegal "machine guns."

An easier solution might be for Congress to enact legislation outlawing bump stocks, which could be subject to a separate constitutional challenge.

Danny Cevallos is an MSNBC legal analyst. Follow @CevallosLaw on Twitter.