No threaded barrels.
State law RCW 9.41.010 defines assault weapon to include pistols that meet the following criteria: (vi) A semiautomatic pistol that has the capacity to accept a detachable magazine and has one or more of the following:
(A) A threaded barrel, capable of accepting a flash suppressor, forward handgrip, or silencer;
(B) A second hand grip;
(C) A shroud that encircles either all or part of the barrel designed to shield the bearer's hand from heat, except a solid forearm of a stock that covers only the bottom of the barrel; or. (D) The capacity to accept a detachable magazine at some location outside of the pistol grip;
Further, on page 5 of HB1240 threaded barrels is just one of the characteristics that cause a semi-auto firearm to fit into the State's definition of an "assault weapon"
20(G) Threaded barrel designed to attach a flash suppressor, sound suppressor, muzzle break, or similar item;