All directions given assume that the reader is holding the rifle in firing position.
Fire control group (FCG) Hammer, trigger, disconnector (disco), selector (safety) and their pertinent springs and pins.
Lower parts kit AR X RayFlattop upperUpper 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 barrelForging Part made from compressing a block of metal between forming dies.
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 FSBFront sight base seatArea of the barrel onto which the FSB is mounted.
M4 forged FSBFunction check Process by which the safety and timing of a rifle is checked.
Function CheckGAU-5 seriesGun, Aircraft, Unit model 5. US Air Force designation for their versions of the XM series of weapons.
USAF SP with GAU-5/A/AGAU-5 with A1 flash suppressorGAU-5”Child Soldier” with GAU-5Gas BlockGas Port (GP)Hole in the barrel, underneath the FSB, through which gas is vented into the gas tube.
M4 GPGreen Follower (GF)Improved magazine follower for 30 round magazines. Has longer “anti tilt” legs.
Del-Ton Green FollowerGroup 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” HBARHammer/Trigger pins (H/T) (HT)Interchangeable pins used to hold the hammer and trigger in the lower.
H/T pinsHeight over bore (HOB) Self-explanatory. Most often used when discussing iron sights and optical sights.
Inside Diameter (ID)Self-explanatory.
J springJ 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 failureLength 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” LWMilitary 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.
MilsurpMilitary surplus. Generally used to describe ammunition, sometimes used to describe parts.
M16 lower receiverOriginal design lower lacking the raised fences around the magazine release button. Common on Colt SP1, and later, rifles.
SP1 Colt Sporter II lowerM16 stockOriginal design buttstock having no trap door in the butt and equipped with a pivoting sling swivel. LOP is ~13”.
Moore Militaria M16 butt M16 buttplateM16 upper receiverA1 upper lacking the forward assist. Sometimes called an SP1 upper.
M16 on top SP1 upperM4 barrel14.5” 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 barrelM4 rampsExtended (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.
”4” Stamp on extensionM4 StandardTroy comparison M4 to standardAlbalos M4M4 series carbineLatest standard issue military long arm. Comprised of a collapsible stock, M4 upper with detachable iron sights, 14.5” M4 barrel, and A2 lower.
M4 carbineM4 upperFlattop 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