All you'd have to do is short chamber the rifle, trim your brass to match and avoid this restriction COMPLETELY.  This is another example of BAD LEGISLATION.    

SEC. 7.  Section 12278 is added to the Penal Code, to read:

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.

If it doesn't meet all four of those exactly its not .50bmg!

Whoever added the specs to the bill must have been working on our side, or is a complete idiot.  Probably the latter.

[takes out electronic caliper]
"See officer, my case length is three point nine zero inches - it's not a .50 BMG round!"

"Oh..., then nevermind."

just load it with sabot rounds and you decrease from a 50BMG to say a 45BMG. Smaller projectile and you get around the definition

Obviously getting around the cartridge dimensions will not be difficult. However, it looks like some gunsmiths will be busy rebarreling a lot of rifles. Maybe there will be a temporary glut of barrels chambered in .50BMG on the market.

".50 BMG Ban Increases Sales of SLAP-type ammo"
Talk about your ironic twists.

Actually, after re-reading the bill it says:

a ".50 BMG rifle" means a center fire rifle that can fire a .50 BMG cartridge

so if a rifle can chamber and fire a standard .50 BMG round, it is .50 BMG rifle.
As Mike points out, short chambering seems to be the loophole.

Quoted: ".50 BMG Ban Increases Sales of SLAP-type ammo" Talk about your ironic twists. Actually, after re-reading the bill it says: a ".50 BMG rifle" means a center fire rifle that can fire a .50 BMG cartridge so if a rifle can chamber and fire a standard .50 BMG round, it is .50 BMG rifle. As Mike points out, short chambering seems to be the loophole.

It's not a loophole, it's the letter of the law.