I have read that Franz Ferdinand was wearing body armor when he was assassinated.
That was very rare expensive silk body armor.
That was hardly in use to my knowledge.
Such body armor had existed for quite some time, but was extremely expensive, only the most wealthy of individuals could have afforded it.
Most body armor was used by criminals and was makeshift armor of various materials sewn together in many layers.
Just how much of the .357's effectiveness against human targets relies on bullet expansion?
I would say a significant amount if comparing it to wider diameter defensive calibers.
Obviously shot placement is the key concern.
However there is more penetration than necessary against human targets, meaning you have more recoil and penetration for a .357 dimater round than you need. That means you are putting a lot more energy into making a .357 diameter wound that penetrates to the required depth than necessary.
With reliable expansion it offsets the unnecessary energy and puts it to good use expanding very well.
You end up putting that energy into creating a larger diameter wound, that is more on par with the energy of the round in a man size target. Unlike with many rounds the expansion of the light .357 rounds is very reliable. They impact at such high velocity that the impact itself often begins the expansion.
The expansion can be greatly controlled with the use of various rounds, and you can gain significant penetration or significant expansion, leaving you with an extremely versatile firearm.
The penetration available is so high that you can use the .357 Magnum to take most game if you really wanted to with rounds that are very heavy and do not expand. Of course shot placement becomes even more important than with a larger caliber.
Or you can reduce that penetration as much as possible with rapidly expanding light ammo at high velocities and have a very effective man stopper.