Atom if you do some researching you'll find not many 12mm/.50 cal bullets had much of anything in them other than tracers, although some were made to be Armor Piercing Incendiary. So that if they hit a enemy fuel tank they had a higher chance of starting it on fire.
I do not know of any standard .50 cal mg round that was packed with an explosive charge.
Not enough room to work with, and too unstable for field conditions. Not to mention not enough room for fusing, ignition etc.
A self igniting API round, is much simpler, can be as simple as a Armor piercing bullet, with a hollow in the back containting a slow burning fuse, or White Phosphorus.
Totally different from an exploding 20mm round.
