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You are right on the money. Steel type, alloy and heat treatment make a huge difference in resistance to bullets.

If I remember correctly, H-P does not say in his book that the military calibers DIDN'T penetrate the take-home German loophole plates, but rather that the NE's did. Also, I do not believe he ever published a comparative test between the calibers.

From my own shooting, I will say the standard issue MKVII .303 performs rather poorer against steel plate than many loadings of 7.92x57 and .30-06 possibly due I believe to the varied fillers used in the tip of the MKVII. And certainly 215 grain MKVI would offer no advantage and likely perform even poorer against steel. Early in the War, I am not at all certain that issue stocks of ammo to British troops did not include both types. In fact, I have read references to both types in service during the Great War.

A heavy caliber NE bullet might offer advantages against cast iron which is very brittle indeed.

All plate shooters know the effects of continued impact. I have a piece of 3/8 inch plate we used here for 300-500 yard shooting. It stopped all sub-.50 BMG calibers for a long time but eventually developed stress cracks and then started to let individual rounds go through, sometimes "squirting" thru in a flattened shape. Even the "poor" .303 got so it would hole it from time-to-time.

For the most dramatic of example of that, see page 355 of the War Office Textbook of Small Arms 1929, with picture facing page 354. This 2-inch thick plate was holed by continuous impact of rounds at 500 yards. Two-inch thick steel plate is a lot of steel plate.

H-P also mentions the use of a .350 Mauser for sniping purposes though the reference I am thinking of involved its use to attract fire due to its muzzle flash and smoke. Assuming soft point ammo was used, a center mass strike from this thing would have been a classic example of a combat multiplier.

Finally, armor penetration is now a science. It was more of an art when Hesketh-Pritchard wrote his book.

Shooting plate of unknown alloy, hardness, shear strength etc is interesting but not very instructive for military purposes. Just because one bullet at one velocity penetrates one type of plate better than another bullet, doesn't mean that the latter bullet won't beat the former in some other type of plate. Maybe there are some steel, iron, or alloy types that the big NE's will hole better than smaller rounds.

Maybe an engineer can fill us in on "adiabatic shear" and make a prediction as to what type of metal material a given bullet from a .475 No 2 might defeat that a given bullet from a .303 wouldn't!!