There are several type of Class 3 items that are restricted by the National Firearms Act.
Each state can impose additional restrictions on the sale, purchase, and transfer of class 3 firearms in addition to the compliance that is required with the national Firearms Act.
In California you can own the following items that are regulated the the National Firearms Act
Machine Guns (special permission is required)
Any Other Weapon (AOW) (except Pen Guns or Assault Weapons) *1
Destructive Devices (DD)
Short Barreled Shotguns (SBS) (C&R only)
Short Barreled Rifles (SBR) (C&R only)
In California you cannot own the following NFA restricted items.
*1 AOW’s other than Pen Guns are ok as long as they are not an assault weapon. With the exception of AOW assault weapons that were owned prior to the registrations period are ok. AOW’s are not required to receive a Curio or Relic classification.
Note In California most Class 3 items other than AOW’s must be classified as a Curio or Relic (C&R).
SBS and SBR, that are C&R as well has AOW’s as described above do not require any special state permits.
Permits for Machine guns and DD’s are controlled and at the sole discretion of the DOJ and are rarely issued to civilians or anyone who is not involved in the movie industry.
CA’s Assault weapons laws apply to assault weapons whether they are C&R or not. An assault weapon in CA if meets certain requirements found in the statutes. One of these is semi automatic center-fire rifle with the capacity to accept a detachable magazine that has an overall length of less than 30 inches is an AW. This would mean if you made (through a form 1) a SBR out of an M1 Cabine, it would likely be considered an AW under California law.
Follow this link to find out more about California and NFA restrictions on Class 3 Firearms