In the case of gunshot wounds from unjacketed lead alloy bullets or dirty bullets, a phenomenon known as bullet wipe may be observed, which forms a ring of greasy residue known as a grease or dirt collar that overlays the abrasion collar and is caused by deposits on the skin's surface. Generally, these deposits contain lead from the unjacketed projectile or oil from inside the weapon's barrel, but they may simply be dirty. Studies using high-speed photography have shown that bullet wipe is caused only by the head of the bullet—instead of the body—because the skin recoils away from the bullet as it penetrates due to the force of entry.[1]