Fire control group (FCG)
Hammer, trigger, disconnector (disco), selector (safety) and their pertinent springs and pins. Lower parts kit AR X Ray

Flattop upper
Upper receiver with an integral scope base built into it. Sometimes mistakenly referred to by the Colt term, A3 upper. Flattop on M16A4 Flattop on Colt’s Accurized Rifle

F marked barrel
Colt barrel with an FSB of a different height intended for use with Colt flattop upper receivers. Marked with a stamped F on the left side of the FSB. The F is on the gas tube bracket to the rear of the gas tube roll pin. F marked FSB

Forging
Part made from compressing a block of metal between forming dies. Forging line on front of GI FSB Forging line on inside of GI A2 upper

Free float tube (FF)
Handguard that minimizes exterior contact with the barrel. FF tube on Colt’s Accurized Rifle FF tube on DPMS Panther Bull 16”

Front sight base (FSB)
The triangular fixture mounted on the forward portion of the barrel. Serves as a mount for the front sight post, gas tube, and handguard cap. USGI forged FSB

Front sight base seat
Area of the barrel onto which the FSB is mounted. M4 forged FSB

Function check
Process by which the safety and timing of a rifle is checked. Function Check

GAU-5 series
Gun, Aircraft, Unit model 5. US Air Force designation for their versions of the XM series of weapons. USAF SP with GAU-5/A/A

Gas Port (GP)
Hole in the barrel, underneath the FSB, through which gas is vented into the gas tube. M4 GP

Group
A collection of assemblies.

Headspace (HS)
In rimless rifle cartridges, the distance from the face of the bolt to a pre determined datum line in the chamber. HS varies depending on make, model, and calibre.

Heavy Barrel (HBAR)
Barrel with an OD approaching 1” under the handguards. BFI 20” HBAR

Hammer/Trigger pins (H/T) (HT)
Interchangeable pins used to hold the hammer and trigger in the lower. H/T pins

Height over bore (HOB)
Self-explanatory. Most often used when discussing iron sights and optical sights.

Inside Diameter (ID)
Self-explanatory.

J spring
J shaped spring in the hammer. The J spring retains the hammer pin.

KaBoom (kB)
An unpleasant experience involving partial, or total, destruction of one’s rifle. Often directly resulting from poorly reloaded ammunition. Kris Means’ kB M4 failure

Length of Pull (LOP)
Distance from the front face of the trigger to the rear of the buttplate.

Lightweight barrel (LW)
See A1 barrel. Usually in reference to a 16” barrel.
BFI 16” LW

Military Specification (Milspec)
A set of performance standards an item must meet to qualify for military purchase. Milspecs cover every conceivable item and process. Often mistakenly applied to AR parts.

M16 lower receiver
Original design lower lacking the raised fences around the magazine release button. Common on Colt SP1, and later, rifles. SP1 Colt Sporter II lower

M16 stock
Original design buttstock having no trap door in the butt and equipped with a pivoting sling swivel. LOP is ~13”. Moore Militaria M16 buttM16 buttplate

M16 upper receiver
A1 upper lacking the forward assist. Sometimes called an SP1 upper. M16 on top SP1 upper

M4 barrel
14.5” barrel with M4 feed ramps, F marked FSB, and an annular groove forward of the FSB used to attach an M203 grenade launcher. BFI M4 barrel

M4 ramps
Extended (lowered) feed ramps used on Colt M4 barrels to assist in feeding commercial (non FMJ) ammunition. Colt barrel extensions with M4 ramps are marked on their rear with a “4” at the 3 o clock position. Also, the corresponding cuts in the M4 upper. M4 Standard

M4 series carbine
Latest standard issue military long arm. Comprised of a collapsible stock, M4 upper with detachable iron sights, 14.5” M4 barrel, and A2 lower. M4 carbine