I've been thinking on this subject, largely from a contemporary horror setting POV, and the simplest approach seems to me, like Cyberspace (as mentioned above), rate any armour 1-4 in terms of bullet resistance, and that rating reduces the *size* of the attack. So, we keep the archaic armour (yes, plate won't stop a bullet head on, but it will reduce knicks and glances) 1-10, but add this other dimension. Additionally, the bullet resistant armour could change the critical type from puncture to impact, which would also cut out a lot bleeds.
I'm inclined to just use the sling table for bullets: start at medium for a modern pistol, large for a rifle and very large for a .50 machine gun.
This would also reflect that some types of bullet resistant armour aren't brilliant against other types of attacks.