Really?!? The reverse is true in CA.
Taken from TheHighRoad.org:
In CA, thread barrels are only illegal when installed on semi-auto pistols that use detachable magazines.
Threaded barrel + semi-auto pistol + detachable magazine = illegal [PC 12276.1(a)(4)(A)]
Threaded barrel + semi-auto pistol + fixed magazine = legal
Threaded barrel + revolver = legal
Threaded barrel + single-shot pistol = legal
Threaded barrel + bolt-action pistol = legal
Threaded barrel + lever-action pistol = legal
Threaded barrel + semi-auto rifle = legal
Threaded barrel + bolt-action rifle = legal
Threaded barrel + lever-action rifle = legal
Threaded barrel + pump-action rifle = legal
Threaded barrel + single-shot rifle = legal
Threaded barrel + semi-auto shotgun = legal
Threaded barrel + pump-action shotgun = legal
Threaded barrel + lever-action shotgun = legal
Threaded barrel + single/double barrel shotgun = legal
Penal Code 12276.1
(a) Notwithstanding Section 12276, "assault weapon" shall also mean any of the following:
(1) A semiautomatic, centerfire rifle that has the capacity to accept a detachable magazine and any one of the following:
(A) A pistol grip that protrudes conspicuously beneath the action of the weapon.
(B) A thumbhole stock.
(C) A folding or telescoping stock.
(D) A grenade launcher or flare launcher.
(E) A flash suppressor.
(F) A forward pistol grip.
(2) A semiautomatic, centerfire rifle that has a fixed magazine with the capacity to accept more than 10 rounds.
(3) A semiautomatic, centerfire rifle that has an overall length of less than 30 inches.
(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.
(6) A semiautomatic shotgun that has both of the following:
(A) A folding or telescoping stock.
(B) A pistol grip that protrudes conspicuously beneath the action of the weapon, thumbhole stock, or vertical handgrip.
(7) A semiautomatic shotgun that has the ability to accept a detachable magazine.
(8) Any shotgun with a revolving cylinder.