"Hard" body armor contains one or more hard plates carried in a Kevlar garment. The hard plate is normally made of high tech ceramic material, although some older plates could be made of AR500 armor steel. Steel plates in hard body armor are obsolete these days because of their excessive weight. The hard plates normally cover the front and rear of the torso only. There are some exotic armors such as Dragon Skin that have dozens of small overlapping hard plates that give wider coverage.
If a bullet misses the hard plate and hits only the Kevlar garment, it is encountering only soft body armor and will behave accordingly.
The primary mechanism by which bullets penetrate hard or soft armor is velocity, with bullet construction coming in second. A small, soft bullet travelling at 4,000 fps is more likely to penetrate hard body armor than a large solid bullet at 2,000 fps. Ceramic plates are lighter than steel, but can be brittle and may not withstand multiple hits.
Soft body armor normally stops projectiles that travel no faster than about 1,500 fps. Some AP bullets will do the trick at lower velocities, and some large bore projectiles such as shotgun slugs will be stopped by soft armor at somewhat higher velocities.
I have personally fired .22 RF Magnum rifle bullets (at about 1,800 fps) that went through soft body armor that contained 18 layers of Kevlar, and then fired 12 gauge Foster type slugs at the same armor and had the slugs stopped by the vest.
Soft body armor can be easily penetrated by sharp objects such as knives, but is useful against slashing attacks. There is special soft body armor that contains a mesh that does protect against edged weapons.
I have fired several arrows from compound bows at soft body armor and have never gotten a penetration, but I don't doubt that it could happen with certain razor style broadheads. Also, I have never heard of a documented case where someone was killed by blunt trauma from a vest that stopped a bullet. I would be interested in learning of any authentic cases. I am not claiming that blunt trauma cannot cause injury such as bruising or even a cracked rib, but I've never heard of a documented death.
I have 1/2" and 5/8" AR500 steel targets on my range at camp and have never had a .458 Winchester Magnum 500 gr. bullet do more than scratch the surface of the plates. However I will not fire my .220 Swift at those same plates at less than 200 yards for fear of penetrating or damaging them.
The ability of hard plates and armor steel to stop bullets varies with the angle of the impact and whether the plate can move when struck. The greater the angle and the more the plate can move or swing when hit, the better it will stop the bullet. These principles also apply to soft body armor.
Mild steel is another story. Lots of bullets that penetrate soft boiler plate will be stopped cold by a thinner sheet of genuine armor steel.
Thanks for the elaborate response. I'm not worried about soft armor, I was wondering if it could go thru hard armor.. the one designed to stop .308's , 5.56's, ak's, and 30-06's. I have a questions if you don't mind. Now is the ceramic plate better at stopping bullets then steel, or do they just use it because it's cheaper and lighter? Also, does the ceramic plates absorb the shock as well as steel? I can imagine that bullets just bounce off of steel while ceramic cracks and absorbs the bullet created more shock to the wearer. Also, would a terrorist like say al queda or some of these other scumags, practically they would probably get the softer armor or the harder armor? I mean i can't imagine them getting hard armor just because of the costs and weight, but at the same time, no one imagined they coudl fly a plane into the world trade center
