Restrictions on semiautomatic handguns
From Calguns Foundation Wiki
California also classifies semiautomatic handguns with certain features as "assault weapons"
As of Jan 1, 2012, the referenced law is Penal Code 30515
12276.1. (a) Notwithstanding Section 12276, "assault weapon" shall also mean any of the following: (4) A semiautomatic pistol that has the capacity to accept a detachable magazine and 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 his or her hand, except a slide that encloses the barrel. (D) The capacity to accept a detachable magazine at some location outside of the pistol grip. (5) A semiautomatic pistol with a fixed magazine that has the capacity to accept more than 10 rounds.
Unlike rifles, there is no requirement for pistols to be centerfire, only semiautomatic.