Femur or acetabular dysplasia can lead to femoral acetabular impingement (FAI). Subacromial bursitis often presents with a constellation of symptoms called impingement syndrome. Many causes have been proposed in the medical literature for subacromial impingement syndrome. The combustor is an impingement cooled annular design. It is used to distinguish between anterior instability and primary shoulder impingement. The scapula plays an important role in shoulder impingement syndrome. Conductive hearing loss is usually concomitant with impingement of abnormal bone on the stapes footplate. Another local chip cooling technique is jet impingement cooling. The Stoner Weapons System used a piston-operated gas impingement system. In 1983 Neer described three stages of impingement syndrome. The SR-25 uses a rotating bolt and a direct impingement gas system. They are further divided into primary or secondary causes of impingement. It not only supports the diagnosis for impingement syndrome, but it is also therapeutic. Currently there are two techniques&colon; micro-channel heat sinks, and jet impingement cooling. The Mk47 relies on a direct impingement gas system.