Subcutaneous emphysema is often found in car accident victims because of the force of the crash. Chest trauma, a major cause of subcutaneous emphysema, can cause air to enter the skin of the chest wall from the neck or lung. When the pleural membranes are punctured, as occurs in penetrating trauma of the chest, air may travel from the lung to the muscles and subcutaneous tissue of the chest wall.
Both the Army and the Marine Corps teach that the maximum effective range of the M16A2/A3/A4 is 500 meters on a point target (individual enemy) and 800 meters on an area target (i.e. troops in the open).
At a measured distance of 100 yards, set up a 4 foot tall target with the aim point near the bottom. Using a level, draw a vertical line extending from the aim point to the top of the target. Fire a 5 shot group at the aim point with your 100 yard zero. Adjust the scope up 10 MOA and fire another 5 shot group. Adjust the scope up 10 more MOA and fire another 5 shot group.