All directions given assume that the reader is holding the rifle in firing position.
Original USGI barrel design having a front sight base seat of 0.625”. Except for the portions over the chamber and directly behind the gas port, the barrel is approximately 0.65” OD. BFI 16” A1
A1 lower receiver
Second design lower, with raised fences around the magazine release button and beneath the ejection port. A1 on left
A1 on left
A1 on top
Second design stock, having a trap door in the butt for cleaning supplies and a fixed sling swivel. LOP is same as that of the M16 stock. M16A1 Parts diagram
A1 upper receiver
Second version of USGI upper with short range and long range apertures. Rear sight is not adjustable for elevation. A1 upper has a forward assist but no brass deflector. Sometimes called a field sight upper. M16A1
Second USGI barrel design having a front sight base seat of 0.750”. Except for this dimension, the A2 barrel closely parallels the contour of the A1. Sometimes mistakenly called an HBAR. BFI 20” A2
A2 lower receiver
Third design lower, with A1 improvements and additional reinforcing around the pivot pin boss, the receiver bridge, and the web behind the takedown pin. Most noticeable difference between A1 and A2 lowers is the raised takedown pin detent spring tunnel of the A1. Raised tunnel is located behind the takedown pin on the right side of the A1 lower. A2 on right A2 on right A2 on bottom
Third design stock with A1 stock improvements, stronger plastic, squared, checkered buttplate, and an extended LOP of ~13.6”. A2 stock on M16A4
A2 upper receiver
Third version of USGI upper with fully adjustable (300 to 800 metres of elevation) rear sight, brass deflector, and forward assist. Sometimes called an adjustable sight upper. A2 on bottom
A4 upper receiver
Detachable carry handle version of A2 upper, called A3 by Colt. Used on the M16A4 rifle. M16A4
Abu Dhabi carbine
Progenitor of the M4 carbine. Designed by Colt for overseas sales.
ARmalite model 15. The earliest of the pre adoption M16s. Early M16
A collection of parts.
Cap screwed onto the rear of AR barrels containing the locking lugs, alignment pin, and feed ramps. KMeans’ barrel
The forward, or counter recoil, movement of the bolt carrier assembly.
Big hole upper/lower
Colt style lower/upper requiring a pivot pin of 0.312” OD instead of the USGI size of 0.250” OD.
Big pin lower/parts
Colt style lower, or FCG parts, requiring H/T pins of 0.170” OD instead of the USGI size of 0.155” OD.
Bolt over base
Malfunction in which the bolt jams open part way up the topmost round in the magazine.
Barrel with very little, or no, taper. Most AR bull barrels approach 1” OD for their entire length. DPMS Panther Bull 16”
A1 upper with a brass deflector. In use by Canadian Forces. OAI C7 Diemaco C7 photo
Early designation for Colt made M16s. See Colt’s Automatic Rifle.
A rifle shorter than the issue rifle. Carbine
Part made by pouring liquid metal into a mold.
Colt’s Automatic Rifle (CAR)
Often mistakenly applied to the first shorties produced by Colt, the XM177 series. Also, Colt’s Accurized Rifle.
Colt’s designation for their M16 carbines. Colt shorty
Cycle of Functioning
Feeding, Chambering, Locking, Firing, Unlocking, Extracting, Ejecting, and Locking.
Two live rounds attempting to feed into the chamber simultaneously. Usually caused by a bad magazine.
Timing malfunction involving firing more than one shot per pull of the trigger. Usually caused by worn FCG parts.
A fixed stock with a shorter than normal LOP. LOP is usually equal to that of a fully collapsed carbine stock. See also Stubby stock. J&T Stubby
Failure to eject (FtEj)
Expended cartridge case successfully extracts (see FtEx), but remains in the upper receiver.
Failure to extract(FtEx)
Expended cartridge case is not pulled completely from the chamber. Extraction ends when the case mouth clears the rear of the chamber.
Failure to feed (FtF)
Live cartridge does not make it past the feed lips of the magazine. Often seen as a “3 point bind” with the nose of the bullet in the chamber, the wall of the case resting on the barrel extension and the base of the cartridge stuck in the magazine.
Failure to lock(FtL)
Round has left the feed lips of the magazine but the bolt is not completely shut.