SAN FRANCISCO, Jan. 3 - California has become the first state to ban a powerful .50-caliber long-range rifle that gun control advocates portray as a military firearm that could easily fall into the hands of terrorists bent on assassination or shooting down an airplane. Under the ban, which was signed into law by Gov. Arnold Schwarzenegger in September and took effect on Jan. 1, it is now illegal to manufacture, sell, distribute or import a weapon known as the .50-caliber BMG, or Browning machine gun rifle, a single-shot weapon widely used not only by law enforcement officers and the military...