An abrasion collar, also known as an abrasion ring or abrasion rim, is a narrow ring of stretched, abraded skin immediately surrounding projectile wounds, such as gunshot wounds. It is most commonly associated with entrance wounds and is a mechanical defect due to a projectile's penetration through the skin. It is caused by a temporary over-stretching of the skin surrounding the projectile's point of penetration. Like all skin abrasions, the abrasion collar tends to dry out due to scraping away of the skin's outer layers and the collapse and dehydration of the underlying cells; it therefore becomes easier to discern with time. This defect is most often seen around rifled firearm entrance wounds due to the striations or grooves in the bullet's surface caused by the rifling on the inside of the weapon's barrel; however, certain other high-velocity projectile wounds can also have the same effect. An abrasion collar is usually found in association with a contusion collar of bruising caused by damaged blood vessels in the skin by hydrostatic forces from the bullet's entry. An abrasion rim defect is also possible in firearm exit wounds under certain circumstances, such as if the skin at the exit was crushed between the outgoing bullet and an unyielding object pressed against the skin over the exit site, or if the projectile exits at an extreme angle. Careful examination of the wound under magnification may show signs of everted (outward-turned) edges characteristic of an exit wound. Multiple projectiles impacting in close proximity together, such as in a close-range shotgun blast, will usually still produce an abrasion rim or artefact, though the wound will likely be irregular in shape. Factors and characteristics Bullet wipe 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. Angle of trajectory The bullet's angle of trajectory at the