be engaged with a bare thumb, index finger, or thumb nail because of the lack of this surface. Their fire-selector was also changed from upward = safe, backward = semi-auto and forward = full-auto, to the now familiar forward = safe, upward = semi-auto, and backward = full-auto of the M16 rifle. The only major difference between the 601 and 602 is the switch from the original four grooves, right-hand 1:14-inch (1:355.6 mm or 64 calibers) rifling twist rate to the more common four grooves, right-hand 1:12-inch (1:304.8 mm or 54.8 calibers) twist. This was done as the original 1:14-inch twist rate to just stabilize the 55 grain bullet used in the 5.56 Ball M193 cartridge under unfavorable conditions could induce too much yaw and due to the resulting excessive in flight bullet destabilization become inaccurate. The one turn in 12 inches increased accuracy and was optimized to adequately stabilize the M193 ball and longer M196 tracer bullets. Design details The ArmaLite AR-15 internal piston action was derived from the original ArmaLite AR-10 action and was later used in the M16 rifle action. This internal piston action system designed by Eugene Stoner is commonly called a direct impingement system, but it does not utilize a conventional direct impingement system. In , the designer states: ″This invention is a true expanding gas system instead of the conventional impinging gas system.″ The gas system, bolt carrier, and bolt-locking design were novel for the time. The AR-15 is a modular weapon system. It is easy to assemble, modify and repair using a few simple hand tools, and a flat surface to work on. The AR-15's upper receiver incorporates the fore stock, the charging handle, the gas operating system, the barrel, the bolt and bolt carrier assembly. The lower receiver incorporates the magazine well, the pistol grip and the buttstock. The lower receiver also contains the trigger, disconnector, hammer and fire selector (collectively known as the fire control group). The AR-15's "duckbill" flash suppressor had three tines or prongs and was designed to preserve the shooter's night vision by disrupting the flash. Early AR-15's had a 25-round magazine. Later model AR-15s used a 20-round waffle-patterned magazine that was meant to be a lightweight, disposable item. As such, it is made of pressed/stamped aluminum and was not designed to be durable. The AR-15's most distinctive ergonomic feature is the carrying handle and rear sight assembly