[Myrddin]

Many black powder rifles are .50 caliber or slightly larger. Was the legislation crafted to just attack the 50 BMG or did it manage to emcompass black powder rifles via vague wording?

[ArmstedFragg]

Here's the definition:

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.

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Oddly enough, it appears that a true 50 caliber weapon (.50 bullet) would be exempted.