After reading the various Supreme Court decisions regarding the 2nd amendment, it seems that a challenge to the .50BMG ban could be brought on the following basis:
First, that a .50BMG rifle is a weapon consistent with being part of a militia, of which all able bodied males (at least) are a part of. It is a weapon in common use at this time in the waging of war.
Second, SCOTUS in US v Miller makes reference to Aymette v State, implying that the criteria in Aymette v State is still considered valid. The court in Aymette v state made the distinction between keeping and bearing, affirming that while the state may regulate the manner in which a weapon may be born, it cannot limit the keeping of weapons consistent with service in a militia.
From reading Aymette, it seems clear that a legislature has a right to regulate the carrying of a weapon, but not the ownership. In other words, the legislature could require that the .50BMG rifle be transported disassembled between the home and the range, for example, but it cannot ban the ownership of such weapon.
It would seem to me that by challenging the restriction on the ownership of a .50BMG riffle, it could be clearly established that the ownership of weapons consistent with the needs of a militia is an individual right, the same way that hiller declared that the ownership of self defense weapons is an individual right.
Since, as far as I know, the .50BMG rifle has never been used in a crime, it would be hard for the CA DOJ to explain why a simple restriction on how to transport such a weapon would not satisfy the stated purpose of protecting the population. The facts do not give the legislature a compelling reason for the outright ban of such weapons.
Having the supreme court declare that the right to own militia weapons is an individual right, would be a step in the right direction.