2 to 4 casing can be found on the dead body of an enemy that was wielding a anti-materiel rifle at the time of their death. Veteran Rangers typically carry the anti-materiel rifle regularly.
When you fire a weapon chambered in the .50 MG round, there is a 25% chance you'll get a casing and a 20% you'll get a casing if you're using the match load round.
Having the Hand Loader perk can double the chances of recovering .50 MG casings.
You'll get 25 .50 MG casings if you buy a .50 MG case box.
Occasionally some traveling merchants will "sell" .50 MG casings (among many other calibers), but they will always be for free. Likewise, if you try and sell casings to a merchant, you will not yield any money.