It's entirely legal to own and use an off-Roster pistol, and for one CA resident to sell such to another, but it's illegal for an FFL to directly sell one to a non-exempt person (principally LEO).
A lot of guns are not on the CA Roster of 'not-unsafe' handguns. For example, made-in-USA Glocks have been ruled by CA-DOJ to be
different from the ones submitted for testing years ago, and thus need the new 'features' - magazine disconnect and loaded chamber indicator and now microstamping - and cannot get on the Roster.
There is a quirk in the Penal Code on the Roster that exempts handguns with certain characteristics from needing to be on the Roster:
(b) Article 4 (commencing with Section 31900) and Article 5
(commencing with Section 32000) shall not apply to
a single-shot pistol
with a barrel length of not less than six inches
and
that has an overall length of at least 10 1/2 inches when the handle, frame or receiver, and barrel are assembled.
So, some creative folks were taking an off-Roster pistol, adding a long barrel, and a 'sled' of a fixed magazine, that allowed loading only a round into the chamber.
Then, having transferred the gun in 'long barrel, single-shot' condition, the buyer would return the gun for gunsmithing, during which the factory barrel would be re-installed, and the fixed mag converted back to removable. The parts ordinarily are returned to the FFL to be re-used for the next SSE (Single Shot Exemption) conversion.
There has been some concern that the back-conversion might be considered 'manufacturing an unsafe handgun', forbidden by law, but the line between gunsmithing an existing gun and manufacturing has been blurry.
The
new law, actually a decent model for a single-purpose bill, adds this sentence to PC:
However, Article 4 (commencing with Section 31900) and Article 5 (commencing with Section 32000) shall apply to a semiautomatic pistol that has been temporarily or permanently altered so that it will not fire in a semiautomatic mode.
Purpose-designed single-shot pistols remain exempt.