Compatibility guide

Which upper/lower family a receiver-mounted part belongs to. AR-15, the two incompatible AR-10 patterns, and dedicated 9mm lowers do not interchange.

How to check: Manufacturer listings state 'DPMS high-profile', 'DPMS low-profile', or 'Armalite/AR-10 pattern'. Handguards and BCGs for large-frame ARs are pattern-specific.

The bolt's bolt-face diameter and extractor must match the cartridge's case head. Barrel and bolt (BCG / action) must agree.

How to check: Barrel chambering determines it: 5.56/.223/.300 BLK/.350 Legend share the standard AR-15 bolt; 6.5 Grendel and 6mm ARC need a Grendel bolt (.441"); 6.8 SPC and .224 Valkyrie need a 6.8 SPC bolt (.422"); 7.62x39 needs its own; .308-family cartridges use the AR-10 / standard bolt; belted magnums need a magnum bolt face.

Cartridges a part is chambered for or feeds. The barrel's chamber and the magazine must share a cartridge.

How to check: Barrel roll-mark / spec sheet chambering; magazine spec 'compatible calibers'. A 5.56 NATO chamber safely fires .223 Rem (list both); the reverse is not true for a .223-only chamber.

The magazine pattern the receiver (or bottom metal) accepts. 9mm AR lowers are either Glock- or Colt-pattern, and the bolt must match because the ejector and feed geometry differ.

How to check: Lower receiver / bottom metal spec sheet names the magazine family (STANAG, SR-25/PMAG 7.62, Glock, Colt SMG, AICS, AK, Ruger BX).

Thread pitch at the muzzle. Muzzle devices only fit their exact thread.

How to check: Barrel spec sheet 'muzzle thread'. Standard: 1/2x28 for .22 caliber, 5/8x24 for .30 caliber, 1/2x36 for 9mm (also 1/2x28), M14x1 LH / M24x1.5 for AKs.

Outer diameter of the barrel where the gas block clamps. The block's bore must match exactly or it won't seal.

How to check: Barrel spec sheet 'gas block diameter' (.625" pencil/lightweight, .750" most AR-15 / AR-10, .875" / .936" heavy profiles).

Distance from the receiver to the barrel's gas port. The gas tube length must match the barrel's port position.

How to check: Barrel spec sheet 'gas system'. Gas tubes are sold by length name.

Collapsible stocks need a carbine receiver extension; mil-spec (1.148") and commercial (1.168") diameters differ. Fixed A2 stocks need a rifle-length tube.

How to check: Stock spec sheet says 'mil-spec diameter' or 'commercial'. Buffer kits name the tube type.

Hammer/trigger pin size. AR-15s use .154"; some AR-10s (and many Armalite-pattern) use .170" large pins.

How to check: Lower receiver spec sheet 'trigger pin size'. Trigger groups are sold in .154 or .170 versions.

Outer shape and screw spacing of a bolt action. Chassis/stock inletting, bottom metal, and scope bases are cut for a specific footprint and length.

How to check: Action spec sheet 'Remington 700 footprint, short/long action'. Chassis and bottom metal list the actions they're inlet for.

The lever-action rifle family a part is made for. Levers, stocks, forends, triggers, sights and receiver rails are cut for one model's receiver; rails also follow its drilled-and-tapped hole pattern, so a rail that fits several models lists each one.

How to check: The part's fit list names rifle models ("Fits Marlin 336, 1894, 1895"; "Henry H009, H010, H024"). Your rifle's model is on the barrel or receiver roll-mark; Henry model numbers start with H.

Marlin lever guns made by Ruger (2022 on, barrels stamped Mayodan, NC) differ from earlier Marlin- and Remington-made rifles (North Haven, CT / Ilion, NY) in places: some stocks, forends, levers and sights fit only one generation.

How to check: The barrel stamp says where it was made: Mayodan, NC is Ruger-made. A part that fits only one generation says so ("for new 1895 Marlins built by Ruger"; "Ilion, NY barrel stamp").

How the barrel attaches to the action. Pre-fit barrels are made for a specific tenon thread and headspacing system.

How to check: Barrel listing names the action it's 'pre-fit' or 'chambered' for (Rem 700 1-1/16"-16, Savage small shank, Remage, Tikka M18x1). Brand pre-fits share the Rem 700 thread but are headspaced to one maker's action — an Impact pre-fit won't headspace on a Lone Peak. Actions list both the thread and their own pre-fit family; brand pre-fit barrels list only their family.

Ruger 10/22 barrels are .920" bull or .750" sporter at the receiver. The stock's barrel channel must match or it won't fit (sporter) / will float loosely (bull in sporter-only stock is impossible).

How to check: Stock listings say 'bull barrel channel' or 'standard/sporter barrel'.

AK stocks attach to the rear trunnion. Stamped and milled receivers use different stock tangs; side folders need a folding trunnion or adapter.

How to check: Stock listing says 'stamped receiver' or 'milled receiver'; folding stocks list the trunnion they need.

Stocks without an integral magazine well are inlet for a specific bottom-metal pattern. The bottom metal must match the stock's inlet: an M5-inlet stock needs M5 bottom metal (Badger, Hawkins, MDT, Grayboe M5), a BDL-inlet stock needs hinged-floorplate BDL bottom metal.

How to check: Stock listings name the bottom metal they're inlet for ("M5 DBM inlet", "BDL hinged floor plate", "ADL"). Bottom metal is sold by pattern (M5 / BDL / ADL) and action length.

Bolt actions and their stocks/chassis are made right- or left-handed. A left-hand chassis is inlet for a left-hand action (bolt handle on the left) and won't accept a right-hand action, and vice versa.

How to check: Left-hand parts are always labeled (LH, Lefty, Left Hand). Anything not labeled left-hand is right-handed.

How the optic or mount attaches to the rifle. Picatinny-mount optics and scope mounts need a rail; AK side-rail mounts need a side rail on the receiver. A mount lists what it clamps to plus any rail it provides on top (an AK side mount with a Picatinny top is both), so an optic matches the mount it sits on.

How to check: Optic includes or requires a mount; receivers / scope bases list rail type; scope mounts list what they clamp to and any rail they provide.

Rings and one-piece scope mounts clamp a specific main-tube diameter. A 30mm scope needs 30mm rings; a 1" scope won't be held by 30mm rings without inserts.

How to check: Scope spec sheet 'main tube diameter'; mounts and rings are sold by diameter (1", 30mm, 34mm, 35mm).

Mini red dots bolt to a plate or mount cut for their footprint. The common patterns don't interchange without an adapter plate.

How to check: Red dot spec sheet names its footprint (RMR, Aimpoint Micro/T-2, DeltaPoint Pro, RMSc, ACRO); mounts and risers are cut for one.

Most AR handguards bolt to their own barrel nut on a standard upper. Some uppers instead have an integral handguard interface (Aero M4E1/M5E1 Enhanced) that only takes that maker's matching handguards, and those handguards won't mount on a standard upper.

How to check: Handguard listings say 'compatible with M4E1 Enhanced upper only' or 'includes barrel nut'. Uppers with an integrated mount name the handguards they accept.