A few (actually more than a few at this point) years back my cousin made up a target out of five 1/4" thick steel plates and he and I took turns blasting it with our deer rifles.
I was shooting a Ruger 77 in 30-06 and if you hit twice in the same place it would punch through. Single hits with the lighter bullets (150gr soft points) would put a pretty good divit in the steel but would not penetrate. If I remember correctly there wasnt much difference with the 180gr bullets. If I memory serves, the '06 would get thru about 3/4" - 1" before it stopped.
My cousin's .338 Winchester (remington 700 BDL rifle) would cut big hairy holes thru the target.... .338in entrance hole, .75-1.25" exit hole. No Joke.
The moral of this story is if you make enemies with someone who owns a .338 Winchester, First, you have made a mistake, and second you need about two inches of plate steel to hide behind.
As for your Mini 30, I would say that it will definitely go thru a 1/4" of plate steel (mild steel, like common A36 carbon steel) but I give it only even odds of making it thru half-inch thick plate.
This sounds like the kind of thing that the guys over at Box of Truth might be able to answer. Look them up on Google. Great website.