[gordon]

A Brinell Hardness Number ( BHN value ) will give you a decent measure of ballistic penetration resistance up to about 3/8" thick plate, since I'd expect an impression made by a 10mm Tungsten ball.

Over about 3/8" thickness, and depending on the plate production method, the physical characteristics of the metal at the centre of the cross section would be such that a BHN figure would just be a guide, and some chunkier form of testing would be required.

If I was setting it up, I'd have the plate subject to a a drop test, where a known weight was dropped on it from a known height on a particular shape and cross section of pin. It would be reasonably easy to simulate the energy input and physical characteristics of a bullet that way, and the resulting dent would look fairly similar. The result wouldn't be recorded as a pass or fail either, you would record the displacement ( depth ) at the centre of the dent, and make a separate note of any physical damage such as cracking or penetration.

Test frequency would be one or more per type of plate per heat treatment batch and per cast number ( distinguishing between the cast number, which sets the chemical composition, and the heat number, which sets the physical heat treatment characteristics )

If I had to guess, on the 81047 image above, I'd say that someone has done a drop test on the plate, and then a Brinell Hardness at the centre of the drop test mark, probably to generate consistent records as you would know both figures came from the same area of the plate.

If I had to fake up a test report;

Date: 01/01/19xx
Tester: MLU
Cast: ABC123 Bloggs Foundries
Heat: DEF456 Smiths Heat Treatment
Plate Batch: 81047 Production side plate, 5/16" thick
Impression Depth: 3/32" from 50lb load dropped 3 feet on 1/2" standard pin
BHN: 235 using 10mm Tungsten indenter at impact centre.
Notes: No evidence of cracking or penetration