Washington joined a handful of other states this week to ban anyone under 21 from buying a semi-automatic assault rifle.
Voters passed a sweeping firearms measure in November that has drawn court challenge from gun-rights advocates.
The ballot initiative seeks to curb gun violence by toughening background checks for people buying assault rifles, increasing the age limit to buy those firearms and requiring the safe storage of all guns.
This follows a move by the Justice Department last month to ban bump stocks, the device found on many of the weapons used in the Las Vegas shooting.
Nevada governor-elect Steve Sisolak said he will also pursue bans on assault rifles and tougher gun legislation in the state. Sisolak will be sworn in Monday, January 7.