Most military shields that mount up on a cupola, such as an M113, M114, early HMMV are made out of either T1 or M2 plate. Both are a tempered/hardened plate steel. Not the plain AISI-SAE 1018 steel you would buy if you went to the local steel supply place and bought a piece of ordinary plate steel.
If anyone ever wants to weld on this plate, there are specific welding procedures you will need to follow. Not for maintaining its bullet-resistence, but to keep the weld (and surrounding area) from crystalizing, and possibly having a very heavy chunk of metal crashing down on top of someone or something.
I think,(in my advanced years the memory has its limitations), they were designed to stop up to .30 cal. ball. Plain commonly found plate steel, such as the 3/8 mentioned before would barely shine up a ball round up to a couple of hundred yards! An .06'with a softpoint hunting bullet will poke a clean hole in a piece of that plate.
Forget what you see on TV. I used to teach Hunter Education, and would demonstrate the power those rounds have.