The executive branch cannot change the law

Trump’s attempt to ban bump stocks will likely fail without congressional support

Neither he nor his Justice Department can change the legal definition of a machine gun by simply lying about it with a straight face, though I quite understand that lying is always Trump's first thought, and the first thought of his entire administration.

“Today the Department of Justice submitted to the Office of Management and Budget a notice of a proposed regulation to clarify that the definition of ‘machine gun’ in the National Firearms Act and Gun Control Act includes bump stock type devices, and that federal law accordingly prohibits the possession, sale, or manufacture of such devices,” Justice Department officials wrote in a statement on Saturday.

That "clarification" is a lie that will not withstand even the first court challenge.

A machine gun, as defined in the NFA, is "Any weapon which 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 NFA term machine gun refers to all firearms capable of full automatic fire and includes true machine guns, assault rifles, battle rifles, submachine guns, and machine pistols. 

The frame or receiver of a machine gun, and any combination of parts intended to make a machine gun, is legally defined as a machine gun.

For example, according to the ATF, "A Glock conversion switch is a part designed and intended for use in converting a semi-automatic Glock pistol into a machine gun; therefore, it is a "machine gun" as defined in 26 U.S.C. 5845(b)."

Parts that can be used to convert a semi-automatic firearm to fully automatic capability are regulated as machineguns and must be registered and tax paid under the NFA. 

The US Military issued kits T17 and T18 to convert the M1 carbine to an M2, capable of fully automatic fire; these kits are legally "machine guns".

Congress could, as this news piece suggests, change the legal meaning of "machine gun" to cover any weapon with a rate of fire at or above some specified threshold.

A threshold not too high to ban bump-stocks.

Or it could leave "machine gun" alone and just ban devices that raise a firearm's rate of fire to or above such a threshold.

Or both.