Bullet Proof
Making something bullet proof is like making it fool proof.
And the moment you make something fool proof, they make a better fool to prove it.
For fools, there is ALWAYS a chance of human error, and if you don't think there is, you will find out the hard way.
For bullets, there's always a bigger bullet. Or a cop killer.
First off for the basics, most armour used in the military is NOT designed for bullets. Flak vests are - oddly - designed for flaks. Steel helmets are - also designed for flak. Basically, fragments. Even if you don't use fragmentation rounds, explosions almost always create fragmentation. Even a mote of dust can blind you if launched by an explosion.
And the reason most armour is designed for flak instead of bullets is that it's easier. We can make armour that can stop bullets in close range, but it weighs 60lbs. There is heavier, theoretical armour. It works, but it's so heavy it has to be balanced with it's own skeleton to transfer the weight to the ground.
When talking about bullet proof, we have to talk about which bullet.
Rifle bullets are typically smaller, with more propellant. They don't hit as hard, but they are accurate. Pistol bullets are typically heavier, with much less propellant. This impart more force, (stopping power), on the target, but don't have the velocity for penetration.
What this means is that you can, indeed, create armour that stops pistol bullets in close range. Flak vest and steel helmets will typically stop rifle bullets at significant range, (200-300m is the standard accurate range for assault rifles), but in close, no.
The problem is that the bullet isn't stopped by a forcefield. It's stopped by the armour. This means that if it doesn't penetrate, all of it's force is applied, directly to the armour, and thence the wearer.
You still take the punch.
And now there are cop killers. Cop killers are the grossly informal term for steel-cored bullets. Most bullets, still, use lead cores. Lead is the heaviest stable element, and it's down enough that it will import most of it's force to the target.
Steel cores don't flex when they hit something, they typically penetrate. As such, they are typically illegal, even if you want something cheap for range practice that won't spread toxic heavy metals about. With them, even pistols can often penetrate pistol rated armour.
And this is why most soldiers do NOT wear armour. Most infantrymen never wear body armour, and only occasionally wear helmets. Because camouflage and entrenchment is much more effect, and weighs a lot less.
But, in civilian usage, you might not have to hike for a dozen kilometres with a 100lbs rucksack. You might be engaged in home defence. You might be part of an armoured car detail. Either way, you don't have to bear the weight of your armour, your chair does. And almost all crime is with far more concealable pistols rather than much more obvious an cumbersome assault rifles. So, what this means is that body armour is much more effective, and if you ever have to shoot, your body armour will likely save your life. in this case, there is really no middle ground. If you get shot, and you are not wearing armour, you could very well die. If you are wearing armour, you could very well survive.