Flattop upper marked with a “4” above the gas tube cloverleaf and two M4 ramps cut into the upper aligned with the bottom of the feed ramps in the barrel extension. Has all A2 upper improvements. When assembled, the upper unit has an M4 barrel and double heat shield handguards. Colt M4 upper
National Match (NM)
Government term for items intended for use on military competition rifles. There are no true NM parts for the M16 as there is no military NM M16.
Colt designed hammer having a sharp notch on the upper front corner. Intended for use with slotted bolt carriers to prevent full automatic modifications. Colt notched hammer Hammer comparison
Overall Length (OAL)
Self-explanatory. Most often used when discussing barrel lengths or cartridge dimensions.
Outside Diameter (OD)
Standard accessory mounting rail used on USGI weapons. Sometimes called M1913, SWAN, or STANAG rail. Picatinny rail on flattop and RIS/RAS of M16A4
Front pin holding the upper and lower receivers together. Pivot pin
Point of Aim (POA)
Point of Impact (POI)
The rearward movement of the bolt carrier assembly.
Rail Adapter System (RAS)
Free float tube with multiple Picatinny rail attachment points for optics, lights, lasers, etc. KAC RAS kit
Rail Integrated System (RIS)
See RAS. RIS/RAS on M16A4
The forward most portion of the trigger. The sear face interfaces with the lower hammer notch.
Barrel with less than 14.5” of rifled bore. Usually in the 10”-12” range, or an XM barrel. Also, carbines using these barrels. Early Colt shorty
Failure of the bolt to fully recoil to the rear of the top most round in the magazine after unlocking.
Shrouded bolt carrier
Bolt carrier that has no slot cut into the bottom. The firing pin can only be seen from the rear. Biggerhammer shrouded BC
Unintentional detonation of a round when the bolt slams shut. Usually caused by reloads using incorrect primers.
Slotted bolt carrier
Bolt carrier with a slot cut in the bottom that exposes the firing pin. See Notched hammer. Biggerhammer Slotted BC
Special Operations Peculiar Modification (SOPMOD)
A group of items (sights, lasers, lights, etc.) designed for the M4A1 carbines in use by Special Operations Forces. SOPMOD kit
Standardization Agreements (STANAG)
Standards covering the weapons and equipment in use by NATO forces. Intended to insure interchangeability of equipment.
Barrels shorter than 10” requiring handguards and gas tubes shorter than those used on standard carbines. Also, carbines using these barrels. DPMS Stubby
See Entry stock.
Rear pin holding the upper and lower receivers together. Takedown pin
Regarding the FCG, the relationships of the hammer, disco, and trigger.
The rate of rotation of the grooves in the bore. Expressed as a ratio of one revolution to the length of bore necessary for one complete revolution of the bullet. AR15 twists vary between 1X7” and 1X14”.
United States Government Issue (USGI)
Standard issue for US military forces.
US Army designation for the short barreled, collapsible stocked, M16s introduced during the Vietnam War. Often mistakenly called CARs or CAR-15s. XM177
10” to 12” barrels having permanently attached flash hiders bringing their OAL to the legal minimum of 16”. Most resemble the barrels used on the XM177/GAU-5 series. XM barrel close up