The AR-15 is the semi-automatic civilian variant of the M-16, the standard-issue rifle for the U.S. military. The AR-15 is famous both for its accuracy and its modular nature. Components for the AR-15 can be purchased from a number of manufacturers in a number of configurations. If you have bought a custom-built AR-15 from someone else, you may want to be able to identify the manufacturer of the parts used in the build, including the upper receiver. The upper receiver includes the chamber, bolt and charging handle.
Items you will need
AR-15 upper receiver
Flashlight
Point the AR-15 rifle in a safe direction and remove the magazine. Verify that the rifle is unloaded by cycling the charging handle multiple times and visually checking the chamber.
Examine the upper receiver for manufacturer's stamping. Most manufacturers stamp production codes just above or to the left of the ejection port, and occasionally below the rear sight. These production codes indicate the manufacturer of the receiver. For instance, the letters "CAF", "CH", and "CM" all indicate the upper to be of Colt manufacture.
"DK" indicates a Diemaco/Kaiser upper.
"FK", "FS", "BK", "DK", "EK", "FK", "AF", "CW", "AA" or "FS" are usually found on Bushmaster uppers.
A "splintered A" marking is usually found on Olympic Arms uppers.
While these markings alone are not definitive, they are usually a good indication of the origin of the upper receiver on your rifle.
Remove the hand guards and examine the underside of the barrel with a flashlight. There should be information stamped here indicating the manufacturer and rate of twist of the barrel.