Glossary

Terms are grouped alphabetically. Where a term relates to a product category we carry, the entry links to it so you can go straight from definition to parts. If you run into a term that is not listed here, contact us and we will add it.

A

Active Reset

A trigger system in which the rifle's own cycling mechanically pushes the trigger forward to its reset position after each shot, rather than relying on the shooter's finger to release it. Used interchangeably with "forced reset." Each shot still requires a separate rearward movement of the trigger.

Adjustable Gas Block

A gas block with a screw or detent that lets you control how much gas is bled from the barrel to cycle the action. Used to tune a rifle for suppressor use, different ammunition, or to slow the cycle rate on forced reset setups.

AK / AKM / AK-74

The Kalashnikov family of rifles. The AK-47 is the original milled-receiver design in 7.62x39. The AKM is the later stamped-receiver version of the same rifle. The AK-74 is the 5.45x39 variant. Aftermarket furniture is built for either stamped or milled receivers and is not interchangeable between them. See our AK parts and KPYK collections.

Ambidextrous (Ambi)

A control that can be operated from either side of the rifle. Common ambi parts include safety selectors, charging handles, magazine releases, and bolt catches.

AR-9

An AR-pattern firearm chambered in 9mm, typically using Glock or Colt magazines and a blowback action instead of a gas system. Many AR-15 trigger and bolt parts are not compatible with AR-9s because of the different bolt geometry.

AR-10 / LR-308

The larger-frame AR platform chambered in .308 Winchester, 6.5 Creedmoor, and similar cartridges. AR-10 parts are not interchangeable with AR-15 parts except for some fire control and small parts. "LR-308" refers to the DPMS-pattern standard most aftermarket AR-10 parts follow.

AR-15

The semi-automatic, direct-impingement rifle platform most commonly chambered in 5.56 NATO / .223 Remington. Its modular design and standardized mil-spec dimensions are why the aftermarket is so large. See our AR-15 parts collection.

B

Barrel Nut

The threaded nut that secures the barrel to the upper receiver. Free-float handguards mount to the barrel nut rather than the barrel itself. AK handguards similarly clamp around the barrel and receiver rather than the barrel nut.

Barrel Profile

The thickness and contour of a barrel along its length. Common AR-15 profiles are Government (M4), Pencil (lightweight), and Heavy/HBAR. Lighter profiles save weight but heat faster; heavier profiles resist heat at the cost of weight.

BCG (Bolt Carrier Group)

The assembly that chambers, fires, extracts, and ejects each round in an AR-15. It includes the bolt, carrier, firing pin, cam pin, and gas key. See Full-Auto BCG for the difference between carrier profiles. Shop bolt carrier groups.

Buffer

The weighted cylinder in the buffer tube that absorbs the bolt carrier's rearward travel and returns it forward. Buffers are sized by weight: Carbine (3.0 oz), H1 (3.8 oz), H2 (4.6 oz), and H3 (5.2 oz). Heavier buffers slow the cycle rate and reduce bolt bounce.

Buffer Tube (Receiver Extension)

The tube that threads into the rear of the lower receiver and houses the buffer and buffer spring. Comes in mil-spec and commercial diameters, which affects stock fit.

Bufferless

A firearm design that does not use a buffer tube, such as the CMMG Banshee, Sig MCX, or folding-stock AR variants with internal recoil systems. Many AR-15 trigger and reset devices are not compatible with bufferless designs.

C

Cerakote

A ceramic-based coating applied to firearm parts for corrosion resistance, wear resistance, and color. Common on handguards, receivers, and AK furniture. KPYK finishes its AK parts in Cerakote in Armor Black, Coyote Tan, and OD Green.

Charging Handle

The T-shaped handle at the rear of the upper receiver used to pull the bolt carrier rearward. Ambidextrous and extended-latch versions make it easier to operate with either hand or with an optic mounted.

Chrome-Lined Barrel

A barrel with a hard chrome coating inside the bore and chamber for corrosion resistance and longer service life, at a slight cost in accuracy potential compared with stainless steel.

CRC

The model number prefix used on KPYK products (for example, CRC 1U004, CRC 2U002). Every KPYK part carries a CRC number regardless of which brand name a retailer uses.

D

Direct Impingement (DI)

The gas system used by the standard AR-15, in which gas is routed through a tube directly into the bolt carrier to cycle the action. Lighter and simpler than a piston system but runs dirtier.

Disconnector

The part in a standard fire control group that catches the hammer after each shot and holds it until the trigger is released, preventing a second shot from a single pull. Some active reset devices, such as the Hoffman Tactical Trigger Kicker, replace the disconnector.

Dovetail (AK Side Rail)

The standard optic mounting rail on the left side of most AK receivers. Side mounts like the KPYK CRC 2U002 clamp onto this rail to provide a Picatinny platform above the receiver.

F

FCG (Fire Control Group)

The trigger, hammer, disconnector, and their springs and pins, as a unit. A mil-spec FCG has standardized dimensions that most aftermarket trigger devices are designed around.

Feed Ramps (M4 Feed Ramps)

The angled cuts at the rear of the barrel extension and upper receiver that guide rounds from the magazine into the chamber. M4 feed ramps are extended into the upper receiver for more reliable feeding.

Forced Reset Trigger

A trigger system in which the rifle's cycling action forces the trigger forward into reset after each shot. The forced reset trigger is commonly abbreviated FRT. The shooter still fires one round per function of the trigger, which is what distinguishes it from an automatic weapon. NSPEC carries forced reset trigger systems including the Hoffman Tactical Trigger Kicker and the Super Safety. Laws on forced reset triggers vary by state; verify your local regulations before purchasing.

Free-Float Handguard

A handguard that attaches to the barrel nut and does not touch the barrel anywhere along its length. Eliminates pressure on the barrel from grip, bipods, or slings, which improves accuracy consistency.

Full-Auto BCG (M16 Profile Carrier)

A bolt carrier with the full-length rear shroud and trip surface of the M16 carrier, as opposed to a cut-back semi-auto carrier. It is entirely legal for semi-automatic rifles and adds reciprocating mass. Required or recommended for most forced reset trigger systems because the underside trip surface is what unlocks the reset mechanism.

G

Gas Block

The fitting on the barrel that captures gas from the gas port and directs it into the gas tube. Available in fixed, adjustable, low-profile, and railed versions.

Gas System Length

The distance from the chamber to the gas port. Standard AR-15 lengths are Pistol, Carbine, Mid-length, and Rifle. Longer gas systems cycle more gently; shorter ones cycle harder and faster. See our carbine, mid-length, and rifle-length gas tube and block combos.

Gas Tube Cover

On an AK, the upper handguard piece that sits over the gas tube. Railed gas tube covers add a Picatinny section above the handguard for optics or lasers.

H

Hammer (Semi-Auto vs. Demilled Auto)

The part that strikes the firing pin. A true semi-auto AR-15 hammer has a full radius at its tip. A demilled full-auto hammer is an M16 hammer with its auto sear surface ground flat, commonly found in budget lower parts kits. Some reset devices require a true semi-auto hammer and will not function reliably with a demilled one.

Handguard

The forend that covers the barrel and gas system and gives the shooter a place to grip and mount accessories. See Free-Float, M-LOK, and Picatinny.

Heavy Buffer

Any buffer heavier than the standard 3.0 oz carbine buffer, most commonly H2 or H3. Used to slow cycling on over-gassed rifles, suppressed rifles, and forced reset setups. The Odin Works HFRT kit is a tunable heavy buffer and spring combination.

High-Shelf vs. Low-Shelf Lower

Refers to the height of the shelf inside the lower receiver behind the trigger pocket. Some drop-in cassette triggers require a low-shelf lower for clearance. Most single-part reset devices work in either.

L

Lower Receiver

The serialized half of an AR-15 that houses the fire control group, magazine well, and buffer tube. This is the legally regulated component. NSPEC does not sell serialized lowers or complete firearms.

LPK (Lower Parts Kit)

The collection of small parts needed to complete a stripped lower: trigger, hammer, disconnector, springs, pins, selector, magazine release, bolt catch, and pistol grip. Quality varies widely, especially in hammers and triggers.

LPVO (Low-Power Variable Optic)

A magnified scope with a 1x low end, typically 1-4x, 1-6x, or 1-8x. Gives red-dot-like speed at 1x and enough magnification for mid-range shots.

M

Milled Receiver

An AK receiver machined from a solid block of steel, as on Arsenal SAM7 and SLR-101 rifles. Heavier and more rigid than stamped receivers, with different trunnion dimensions that require dedicated furniture such as the KPYK CRC 1A003 and CRC 1A006.

Mil-Spec

Short for military specification. On the AR-15 it refers to parts built to the dimensional and material standards of the M4/M16, which guarantees interchangeability. "Mil-spec trigger," "mil-spec buffer tube," and "mil-spec BCG" all refer to those standard dimensions.

M-LOK

A negative-space accessory mounting system developed by Magpul, using rectangular slots that accept T-nut fasteners. Lighter and smoother than Picatinny rail. Most modern handguards, including KPYK's extended AK rails, use M-LOK.

MOA (Minute of Angle)

An angular unit used to describe accuracy and scope adjustments. One MOA is roughly one inch at 100 yards. A rifle that shoots "1 MOA" groups puts five rounds inside about one inch at 100 yards.

Muzzle Device

Any device threaded onto the muzzle: flash hiders reduce visible flash, muzzle brakes reduce recoil, and compensators reduce muzzle rise. Barrels under 16 inches must have the device permanently pinned and welded to reach legal length unless the firearm is registered as an SBR.

P

Picatinny Rail (MIL-STD-1913)

The standardized rail with cross slots used for mounting optics and accessories. Still the standard on top of receivers and gas tube covers, though M-LOK has replaced it on most handguard sides.

Piston System

An alternative to direct impingement in which gas drives a piston rod that pushes the bolt carrier, keeping gas out of the receiver. Cleaner-running but heavier and often proprietary.

Pistol Brace

A stabilizing device attached to the buffer tube of an AR pistol. Federal regulation of braces has changed repeatedly; check current rules before purchasing or installing one.

R

RPK

The light machine gun version of the AK, with a heavier barrel, reinforced front trunnion, and unique rear receiver geometry. Civilian RPK-pattern rifles and the Russian VEPR use RPK-specific stocks such as the KPYK CRC 5002/9033L.

S

Safety Selector (90-Degree vs. 45-Degree)

The lever that blocks the trigger. A standard mil-spec selector rotates 90 degrees between Safe and Fire. Short-throw selectors rotate 45 degrees. Some reset devices lock against the selector drum and only work with 90-degree selectors.

SBR (Short-Barreled Rifle)

A rifle with a barrel under 16 inches or an overall length under 26 inches. Requires federal registration under the National Firearms Act before it is built or assembled.

Stamped Receiver

An AK receiver formed from sheet steel and riveted to the trunnions, as on the AKM, WASR-10, PSAK-47, and most modern AKs. Lighter and less expensive than milled. Uses stamped-specific furniture such as the KPYK CRC 1U004.

Super Safety

An active reset device developed by Hoffman Tactical that replaces the AR-15 safety selector with a cam and lever assembly, providing Safe, standard semi-auto, and active reset positions. Shop the Super Safety cam and lever set.

T

Trigger Kicker

A single-part active reset device from Hoffman Tactical that replaces the disconnector in a mil-spec AR-15 fire control group. Requires a standard mil-spec trigger, a true semi-auto hammer, a 90-degree selector, and a full-profile carrier. Shop the Trigger Kicker.

Trip Surface

The machined area on the underside of a bolt carrier that contacts the auto sear on an M16 and, on semi-auto rifles, actuates certain reset devices as the carrier returns to battery. Cut-back semi-auto carriers may lack a usable trip surface.

Trunnion

On an AK, the steel blocks at the front and rear of the receiver. The front trunnion holds the barrel; the rear trunnion holds the stock. Trunnion dimensions differ between stamped, milled, and RPK receivers, which is why furniture is receiver-specific.

Twist Rate

How many inches of barrel it takes for the rifling to make one full rotation. A 1:7 twist stabilizes heavier 5.56 bullets (62 to 77 grain); a 1:9 twist favors lighter bullets (55 grain). 1:8 is a common compromise.

U

Upper Receiver

The non-serialized top half of an AR-15 that holds the barrel, handguard, bolt carrier group, and charging handle. Complete uppers can ship directly to your door with no FFL. See our complete upper receivers.