I personally have shot several old junk cars and 22 will go completely through both sides of a car.I don't know what I am talking about, but I've seen youtube videos of AR and AK rounds travelling through the side of a car and still having enough power to punch through a 3/4" sheet of plywood. So I'm not so sure that I would trust thin steel.
I suppose you could always test it at the shooting range with a couple hundred rounds.
I worked with a shield on entry teams too. It will stop a bullet but you will probably get a lot of nasty spall in your face. It takes a lot of practice to use one. I had to hold my pistol "gang banger style" to shoot with one since I'm kinda vertically challenged.I carried the ballistic shield on the entry team. It had a window, a light and would stop up to a .308. It was also as much fun to carry as a Buick.
I know for a fact that 30-06 270 and 308 will penetrate half inch standard cold rolled plate steel.
True that all three of these calibers will do what you say, but when you add that bronze tip that .270 round does some nasty business.
They will not penetrate 3 quarters.