If you buy a .50 upper, ask the manufacturer if it's been upgraded with the hammer/sear interrupt. Same if you buy a used one, ask the owner or get the serial number and ask the manufacturer if it has the upgrade.
The reason for this is due to it being proven that it was possible for the uppers to be fired out of battery. The only upper that didn't was the Ferret50 made by Spider Firearms. Newer uppers should have this added, just check if you buy an older one.
The other issue is ammo, you don't want to shoot some foreign stuff like Dominican Republic "yellow tip". Old Israeli ammo with the Hebrew lettering headstamp. There's a couple others, just can't think of them right now.
American made .50 before '52 is to be considered corrosive. Foreign stuff that you don't have any info on is to be considered corrosive. Stuff foreign made into the late '90's has been found to be corrosive! Some people doubt but the test proved otherwise, buyer beware!
Just because someones reloads are safe in their rifle, doesn't mean it's safe in yours. Don't shoot any ammo you don't know where it came from or how it was loaded.