It is my understanding that there are two factors which primarily account for handgun bullets penetrating soft body armor; velocity and bullet configuration.
The higher the velocity the more likely the bullet will shear the fibers which make up the soft body armor.
By bullet configuration I encompass two factors: one, the three dimensional shape, (pointy is better at putting a cutting point into the fibers which allow the rest of the bullet to follow); and two, hardness of the bullet material. A pure copper bullet with a conical point driven at 1500 fps (regardless of caliber) is likely to defeat a Class IIIA vest.
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