I’m a bit off the original topic, but I think I can add to some of the posts made. Unfortunately, my knowledge is limited to law enforcement usage. This is a bit rambling:
1 – Soft body armor that has been fired into is no longer serviceable and should not be used. It has been weakened in the area where shot and additional rounds in the same area may go through. (A firearm, esp. a sub gun, can eat its way through a vest.)
2 – Kevlar loses up to 20% of its effectiveness when wet. Often Kevlar is given a waterproof coating (making it a little stiffer) or is put in a plastic bag to prevent this. The NIJ tests body armor both wet and dry before giving a rating. When the Kevlar dries out, it regains its full effectiveness.
3 – The Kevlar type (29, 129, etc.) and the number of fabric layers are not the only factors in bullet resistance. Other issues include thread count (the finer the weave, the stronger the material) and to what degree, if any, the layers are cross-stitched together (cross-stitching makes the material stronger but stiffer).
4 – Kevlar doesn’t do a good job of stopping edged weapons, though it’s certainly better than nothing. Prison guards often wear vests of different materials, which sometimes offer no firearms protection at all.
5 – Strangely, Kevlar is strongest when a bullet hits it straight on. An angled shot is more likely to penetrate since it tends to tear the material. If you’re serious about seeing what a Kevlar vest will stop, you should shoot it at an angle. The NIJ added angled shots to its testing protocol in 1985.
6 – It was once thought that Kevlar vests degraded with age. At one time, Dupont (maker of Kevlar) recommended that vests be replaced every five years. It now appears that degradation occurs with use (and misuse), not age. Misuse can include exposure to ultraviolet light and laundry bleach. Misuse MAY include folding the material since it is commonly thought that a hard fold of the material can pull the fibers apart slightly, weakening the material.
7 – Individuals testing vests have been injured by bullets bouncing back at them. This is a particular danger when someone tries to evaluate backface deformation during testing by putting the vest in front of a material like wet newspapers. The springiness of the material can send the bullet back. (Backface deformation measures the blunt trauma against an individual’s body caused by the strike of the bullet - the NIJ uses clay to test backface deformation.)
8 – Body armor is not bulletproof. Since it’s adoption by U.S. law enforcement agencies, at least nine LEO’s have been killed when rifle rounds penetrated their vests. Thus, a unit of body armor should NEVER be referred to as a “bulletproof vest”, because it isn’t!