The .50 BMG is quite rare due to laws and price, especially in California, where .50 caliber rifles are banned. In the United Kingdom, it is possible to own a .50 BMG rifle as a section 1 firearm.
Although effective in conventional battles, the .50 BMG is not very useful in a zombie apocalypse. The round is large and heavy, as are guns that fire this cartridge. While it can be used for base defense or hunting, in most cases you will probably be better off with a smaller, lighter round like the 7.62x51mm NATO; unless the situation demands that you make a long range shot. In that case, the .50 BMG is a good candidate, provided you can find match-grade rounds. The .50 BMG is less accurate than the .338 Lapua Magnum, but is less affected by the wind and can fire further. Also, the power of the round is sure to knock down a zombie, and can easily punch through human flesh, making multiple kills with one bullet much easier.