Supporters say they also are motivated by the Oct. 31 slaying of Seattle Police Officer Timothy Brenton and the wounding of his partner. Police believe a .223-caliber semi-automatic rifle was used then.
The bill is backed by Seattle's police department, spokeswoman Renee Witt said. Also pushing it is Washington Ceasefire, a nonprofit that seeks to reduce gun violence. The group plans a news conference today to announce the proposal.
The lawmakers who plan to sponsor the bill are Rep. Ross Hunter, D-Medina; Sen. Adam Kline, D-Seattle; and Sen. Jeanne Kohl-Welles, D-Seattle.
The ban would cover semi-automatics designed for military use that are capable of rapid-fire and can hold more than 10 rounds. Semi-automatics designed for sporting or hunting purposes wouldn't be banned.