The high-powered firearms — once banned nationwide — are now the weapon of choice among young men responsible for most of the country’s devastating mass shootings.
The ban comes after multiple failed attempts in the state's Legislature, and amid the most mass shootings during the first 100 days of a calendar year since 2009.
The Washington law blocks the sale, distribution, manufacture and importation of more than 50 gun models, including AR-15s, AK-47s and similar-style rifles. These guns fire one bullet per trigger pull and automatically reload for a subsequent shot. Some exemptions are included for sales to law enforcement agencies and the military in Washington.
The measure does not bar the possession of the weapons by people who already have them.
The law went into effect immediately upon the signature of Gov. Inslee, who has long advocated for such a ban.
"We know this is going to save lives," said Inslee.
When the bill passed the state House in March, Inslee said he's believed it since 1994 when, as a member of the U.S. Congress, he voted to make the ban a federal law.
"There is no reason on this green earth that anyone needs a weapon of war that is designed for one purpose: to shoot, kill and maim mass numbers of people in seconds," he said, when asked about detractors.
The push to overturn the law was underway before Inslee laid pen to paper on Tuesday morning.
The NRA's state director in Washington told FOX 13 that they were already working on a response.
"The NRA will be making an announcement shortly on our fight to challenge this unconstitutional ban," wrote Aoibheann Cline.
Attorney General Bob Ferguson expected a quick-response. Ferguson, who has pushed for a ban on assault weapon sales in Washington for more than half a decade, told a crowd at the bill signing that he has a "pretty good record" against the NRA in court.
He also admitted that there were times he doubted the legislation would get to the finish point, pointing to the father of 2016 Mukilteo shooting victim as the reason he kept pushing for the legislation for seven straight legislative sessions.
"There are so many people here that are not elected officials," said Ferguson. "I think all of us appreciation how much this organization has been tied to grassroots."
Inslee said lives will be saved because of the semi-automatic rifle ban and two other measures approved by the Legislature this session: one that introduced a 10-day waiting period for gun purchases and another to hold gunmakers liable for negligent sales.
Rep. Liz Berry, who championed the 10-day wait period bill, told FOX 13 that the bill was needed as we approach a new centralized background system set to be put in use in 2024. Once in place, Berry said guns could be sold instantaneously. A threat for both mass shootings, and suicides.
"That's mostly because they're so easy to obtain," said Berry, in response to questions about the usage of guns in roughly 50 percent of suicides. "They're so effective in killing people. This bill will implement a 10-day waiting period. It's a cooling-off period, so people facing crisis get a few days to sit and think about what they're doing. We're not going to give you this deadly weapon for this period of time."
Republican state lawmakers have opposed gun control legislation, but the harshest critiques have come over the assault weapon sale ban. Some Republicans contend that school shootings should be addressed by remodeling buildings to make them less appealing as targets and others saying it infringes on people’s rights to defend themselves.
"HB 1240 clearly violates our state and federal constitutions, which is why it will end up in court immediately," Sen. Lynda Wilson of Vancouver said.
The U.S. Congress reinstating a ban on semi-automatic rifles appears far off. But President Joe Biden and other Democrats have become increasingly emboldened in pushing for stronger gun controls — and doing so with no clear electoral consequences.
Nine states including California, New York and Massachusetts, along with the District of Columbia, have already passed similar bans, and the laws have been upheld as constitutional by the courts, according to Washington's Attorney General Bob Ferguson.
In Colorado, lawmakers debated on Wednesday about similar gun measures, but a sweeping ban on semi-automatic firearms faces stiffer odds.
Lawmakers in the Texas Capitol set aside a slate of proposed new gun restrictions without a vote after hours of emotional appeals from Uvalde families whose children were killed last year. The hearing didn’t end until the early morning hours of Wednesday.
During the debate on the Washington state bill, Democrats spoke of frequent mass shootings that have killed people in churches, nightclubs, grocery stores and schools.
Sen. Liz Lovelett of Anacortes said that kids' concerns about school shootings need to be addressed.
"They are marching in the streets. They are asking for us to take action," Lovelett said. "We have to be able to give our kids reasons to feel hopeful."
Another gun-control bill that passed in Washington this session would allow people whose family members die from gun violence to sue if a manufacturer or seller "is irresponsible in how they handle, store or sell those weapons." Under the state’s consumer-protection act, the attorney general could file a lawsuit against manufacturers or sellers for negligently allowing their guns to be sold to minors, or to people buying guns legally in order to sell them to someone who can’t lawfully have them.
A second bill would require gun buyers to show they've taken safety training. It would also impose a 10-day waiting period for all gun purchases — something that's already mandatory in Washington when buying a semi-automatic rifle.
Some gun-control legislation in other states has been struck down since last year’s landmark U.S. Supreme Court ruling, which set new standards for reviewing the nation’s gun laws. The ruling says the government must justify gun control laws by showing they are "consistent with the Nation’s historical tradition of firearm regulation."