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

This depends on your application. If you need them to stop bullets, regardless of deflection, then you can get away with thinner metal than stopping them for something like body armor.

Another aspect of this type of problem is looking at it from an energy dissipation perspective. Catigliano's method or the strain-energy theorem could be applied to the sheet or plate in order to determine the amount of elastic and plastic deflection caused by absorbing the kinetic energy of a projectile. That deformation would then be compared to the percent elongation for the material, and the sizing of the plate in terms of area could be performed. you should find that the thinner the plate, the larger area between supports required to stop the projectile.

Lastly, as the material deflects, shear stress is generated in a plane normal to the path of the projectile. If a material were built or selected that would naturally delaminate as this shear was applied, additional energy could be bled by the plate system.

The answer is not quite as easy as "how thick would stop a bullet?". It all depends.

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.