Ballistic test have discovered many intresting things.
There comes a point in velocity, were a bullet, when added said velocity becomes harder than the target it hits, this is call sectional density.
For example; if you fire a 40 gr, .224 bullet @ 2800FPS (.22 Hornet) it will penetrate twice the amount of ballistic gelatin then a 40 gr .224 bullet @ 3600 fps. (.223 Remington)
Even though lead is harder than soft tissue, it has been discovered once you add enough velocity, the soft tissue at some point will become harder than the lead.
Now you can change factors involved, but for example, a 9mm 124grn bullet fired out of a MP5 will penetrate more than an AR15 in 223.
Factors that you can change are size of projectile, weight of projectile, material used in construction of projectile, velocity, barrel length, and barrel twist.
You can't compare penetration factors when adding a ballistic vest into the test. A ballistic vest is easy to penetrate, just cut through one with a knife. A ballistic vest works on the principal of the bullet twisting. The interwoven layers of kevlar, Zyleema, and Dyleema are there to capture the bullet, by wrapping them up.
A good test to prove this is simple. Take a 60lb compound bow, and you can fire an arrow through a car door, but take the same bow and try and fire it through one layer of carpet. It won't go through, because the carpet acts in the same way the vest does.
Hope that helps.