His tweeted comments come a day before student and teen-led nationwide marches calling for gun policy reform.
Attorney General Jeff Sessions announced a rule change that expanded the definition of machine gun in federal law to include "bump stock-type devices".
"Since your device does not initiate an automatic firing cycle by a single function of the trigger, FTIB finds it is NOT a machinegun under the NFA, 26 U.S.C. § 5845 (b) or the amended GCA, 18 U.S.C. § 921 (a)(23)", Curtis wrote.
Earlier this month, Sessions sent the changed machine gun definition to the Office of Management and Budget, continuing a process that began February 20 when Trump signed an executive order directing Sessions to get the ball rolling on a bump stock ban.
The decision Friday follows a letter sent by GOP senators in October urging the administration to review the decision after a mass shooter used the device in killing 58 people at a country music concert in Las Vegas.
The legal firearm attachments are created to allow semi-automatic rifles to mimic the actions of a rapid-fire, fully-automatic weapon.
The government determined in 2010 that bump stocks couldn't be regulated unless Congress changed the law. Critics of the president's new push think any effort to reverse that ruling will be challenged in court.
In the United States, fully automatic weapons have been banned since 1986, but bump stocks were described by the ATF in 2010 as "a firearm part and is not regulated as a firearm under the Gun Control Act or the National Firearms Act'". After that, if the rule is made final, bump stock owners will be required to surrender or destroy the devices.
Bump stocks are devices that can be installed onto semiautomatic rifles, accelerating the gunfire.
Among their demands was a ban on bump stocks.
"Legislation is the only answer", she said.
The Justice Department's regulation Friday acknowledges the 2017 decision, saying, "Between 2008 and 2017".
"The NRA doesn't back any ban, the NRA has asked the ATF to do its job and make sure that these classifications are consistent", Loesch told ABC's George Stephanopolous.
Officials at the Bureau of Alcohol, Tobacco, Firearms and Explosives - a division of the Justice Department known as ATF - previously found that, since pressure must be applied to the gun from behind in addition to pulling the trigger, the devices evade a ban on machine guns under the National Firearms Act and Gun Control Act.