I'm not sure when the first hollowpoint .357 Magnum cartridges were offered commercially, but my guess is that most of the people who took these to war and were concerned about the Hague Accords used the "metal piercing" ammunition that was loaded by Remington, Peters, and Winchester/Western at various points in time starting in the late 1920s.
Essentially, it was full metal jacket bullet, or a metal capped bullet.
This type of ammo was originally developed in the 1920s and 1930s, during the heyday of the auto bandit period in US history, when police were having to fire on moving vehicles and were finding that their lead .38s just weren't getting the job done.
They were offered in .38 Special, .38 Super, .357 Magnum, and .45 Auto.
A number of people have said over the years that the jacket on many of these was quite thin and probably didn't help much, at all, in getting through the heavy metal in cars of the time.
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