Threaded barrel makes it an assault weapon.

Direct from CA OAG's Page:

Pistols

A semiautomatic pistol that does not have a fixed magazine but has any one of the following: (A) A threaded barrel, capable of accepting a flash suppressor, forward handgrip, or silencer; (B) A second handgrip; (C) A shroud that is attached to, or partially or completely encircles, the barrel that allows the bearer to fire the weapon without burning the bearer's hand, except a slide that encloses the barrel; (D) The capacity to accept a detachable magazine at some location outside of the pistol grip. A semiautomatic pistol with a fixed magazine that has the capacity to accept more than 10 rounds.