I don’t know about the .50 BMG being used today but I sliced open several .50 cal bullets from the WWII era. There was not a bit of lead in them. The brass shell surrounded a stainless steel core. Awesome little things.
I couldn’t find a thing on the material used for a .50 BMG bullet. I have several cartridges but I always thought they were simply copper over lead except for the black tipped armor piercing.
Ok, that would explain how the P-47s could take out a tank from the air.
Steel core .50 cal ammo and of course a bunch of them hitting about the same area at pretty much the same time (the top rear of the tank where the engine compartment is being the preferred aiming point) could result in a "mobility kill" or even lighting off the fuel and setting the tank on fire.