California has strict banns, and regulations on "assault weapons". Most .50 BMG riffles are illegal, there are exceptions for weapons owned before June 1, 1989, and certain models that are listed by the DOJ. These must be registered. Sale and/or transfer of these weapons is illegal. Weapons that are military look alsikes, that are not on the DOJ roster, and can not chamber a .50 BMG can be owned and purchased. There are restrictions on "banned features" these weapons may have.
Active duty military from outside California can legally bring their personal assault weapons to the state. There are requirements for this. Firearms have to be registered with California's DOJ before they arrive, by providing a copy of their Permanent Change of Station orders, along with a copy of the registration form. The military member must also come from a state where private citizens are allowed to own assault weapons.
Some states have restrictions, banns, or require owners to register firearms they conceder to be "assault weapons". The definitions of an "assault weapon" varies widely from state to state. These definitions can include whether the gun is fully automatic, the magazine size it can carry, and others.