What is the minimum thickness of mild steel, spring steel, or aluminium needed to stop bullets?

Question

specifically from 1 nato standard 5.56 mm ammo and .50 calibre ammunition
taking a distnce from the gun of 20 feet, how much thickness in mm of spring steel, black mild steel, aluminium alloy and fibreglass filler would i need to stop 5.56 nato first
and then 50 calibre bullets

Answer 1

Nice Try...

Don't divulge anything to this terrorist!!

Answer 2

At that range the .50 cal would be hard to stop. I am totally guessing here but you would need at least a foot of steel, I don't think that any thing less than a foot of aluminium would have a chance. They use the .50 cal to go threw concrete walls at 300 yards.

Answer 3

I think Ballistic plate would be a far better substance at stopping military calibre rounds. You would need very thick steel to stop those kind of rounds. 5.56 goes through a buick like its tissue paper.

Answer 4

Mild steel (ASTM 1010-1020)
5.56 mm (non armor piercing) will be stopped head on by 3/8 hot rolled plate.
Spring steel (aka high carbon ASTM 4140, 4130, 4340, T-1 steel )
5.56 mm Nato (non armor piercing) will be stopped by 3/16 hot rolled plate.
Mild steel (ASTM 1010-1020)
Cal. 50 BMG 750 Gr. (non armor piercing) will be stopped by 3/4 hot rolled plate.
Spring steel (aka high carbon ASTM 4140, 4130, 4340, T-1 steel )
Cal. 50 BMG 750 Gr. (non armor piercing) will be stopped by 3/8 hot rolled or cast plate.
Forget aluminum..... Really forget F-glass

Now..... Punk...are we using armor piercing? Are we?? Make my DAY...