bullet-proof vest A term that seems to promise more than it can fulfil. No wearable vest is completely bullet-proof. The ‘proofing’ is relative but is often life-saving. Modern protective vests are made of a nylon-like polymer called Kevlar in which aromatic rings replace the nylon hydrocarbon structure. The difference increases the stiffness 16 times over that of earlier nylon protective vests, and a garment made from 20 fine layers of this material can prevent a 9 mm bullet moving at 1200 feet per second from depressing the skin more than 1.7 inches.