.50 BMG rifles
With 2004's AB 50, California classified .50 caliber BMG rifles - and ONLY that precise caliber - as a new kind of 'assault weapon'.
There are no special restrictions on .50 caliber BMG ammunition, and there are no special restrictions on other variants of .50 caliber rifles or ammunition.
The justification in Penal Code 12275.5, such as it is, reads
(b) The Legislature hereby finds and declares that the proliferation and use of .50 BMG rifles, as defined in Section 12278, poses a clear and present terrorist threat to the health, safety, and security of all residents of, and visitors to, this state, based upon findings that those firearms have such a high capacity for long distance and highly destructive firepower that they pose an unacceptable risk to the death and serious injury of human beings, destruction or serious damage of vital public and private buildings, civilian, police and military vehicles, power generation and transmission facilities, petrochemical production and storage facilities, and transportation infrastructure. It is the intent of the Legislature in enacting this chapter to place restrictions on the use of these rifles and to establish a registration and permit procedure for their lawful sale and possession.
The restricted weapons are legislatively defined as
12278. (a) As used in this chapter, a ".50 BMG rifle" means a center fire rifle that can fire a .50 BMG cartridge and is not already an assault weapon pursuant to Section 12276, 12276.1, or 12276.5, or a machinegun, as defined in Section 12200. (b) As used in this chapter, a ".50 BMG cartridge" means a cartridge that is designed and intended to be fired from a center fire rifle and that meets all of the following criteria: (1) It has an overall length of 5.54 inches from the base to the tip of the bullet. (2) The bullet diameter for the cartridge is from .510 to, and including, .511 inch. (3) The case base diameter for the cartridge is from .800 inch to, and including, .804 inch. (4) The cartridge case length is 3.91 inches. (c) A ".50 BMG rifle" does not include any "antique firearm," nor any curio or relic as defined in Section 178.11 of Title 27 of the Code of Federal Regulations.
Note carefully that this is ANY centerfire .50 BMG rifle - this is not restricted to semi-automatic weapons as the other 'assault weapon' definition is.
.50 BMG rifles were added to various places in the 'assault weapon' sections:
12280. (a) (1) Any person who, within this state, manufactures or causes to be manufactured, distributes, transports, or imports into the state, keeps for sale, or offers or exposes for sale, or who gives or lends any assault weapon or any .50 BMG rifle, except as provided by this chapter, is guilty of a felony, and upon conviction shall be punished by imprisonment in the state prison for four, six, or eight years.
12280 continues, to specify the punishment and that there would be a registration period:
(c) Any person who, within this state, possesses any .50 BMG rifle, except as provided in this chapter, is punishable by a fine of one thousand dollars ($1,000), imprisonment in a county jail for a period not to exceed one year, or by both that fine and imprisonment. However, a first violation of these provisions is punishable by a fine not exceeding five hundred dollars ($500), if the person was found in possession of no more than two firearms in compliance with subdivision (a) of Section 12285 and the person meets the conditions set forth in paragraphs (1), (2), and (3): (1) The person proves that he or she lawfully possessed the .50 BMG rifle prior to January 1, 2005. (2) The person has not previously been convicted of a violation of this section. (3) The person was found to be in possession of the .50 BMG rifle within one year following the end of the .50 BMG rifle registration period established pursuant to subdivision (a) of Section 12285.
The registration period has expired: Penal Code 12285
(2) Except as provided in subdivision (a) of Section 12280, any person who lawfully possesses any .50 BMG rifle prior to January 1, 2005, that is not specified as an assault weapon under Section 12276 or 12276.5 or defined as an assault weapon pursuant to Section 12276.1, shall register the .50 BMG rifle with the department no later than April 30, 2006, pursuant to those procedures that the department may establish.
Transfer in CA is extremely limited, parallel to 'assault weapons':
(3) Except as provided in paragraph (4), no .50 BMG rifle possessed pursuant to this section may be sold or transferred on or after January 1, 2005, to anyone within this state other than to a licensed gun dealer, ...
And .50 BMG rifles are subjected to the same transport and places of use restrictions applied to 'assault weapons' in Penal Code 12285:
(c) A person who has registered an assault weapon or registered a .50 BMG rifle under this section may possess it only under any of the following conditions ...