An AR-15 build has more named parts than most people expect — and that list gets longer when you realise many "parts" are actually assemblies of smaller components. But the build process is more modular than it looks. Once you understand what each slot does, the whole thing clicks into place.
This guide walks through every slot in the Battl Builder, in the order you'd typically spec a rifle, and explains what each part does and why you care about it.
Lower Receiver
The lower receiver is the serialised part — the legal firearm. Everything else attaches to it. Lowers are almost universally mil-spec pattern for AR-15 builds, which is why compatibility matters less here than in other slots. Forged 7075-T6 aluminium is the most common material. You'll often buy a stripped lower and assemble it yourself, or buy a complete lower with the LPK and stock already installed.
Lower Parts Kit (LPK)
The LPK is the bag of small parts that lives inside the lower: trigger, hammer, disconnector, safety selector, mag release, bolt catch, takedown pins, and buffer retainer. Stock mil-spec LPKs are reliable and cheap. If you want a better trigger pull, you upgrade the trigger group — but that's a separate slot in the builder.
Trigger
The trigger group is the one LPK component worth upgrading separately. Mil-spec triggers average a 6–8 lb pull with some grittiness. Aftermarket single-stage triggers (Geissele, Rise Armament, CMC) bring that down to 3–4 lb with a cleaner break. Two-stage triggers add a light take-up stage before the break — preferred by many precision shooters. This is one of the highest-ROI upgrades on a budget build.
Buffer Tube, Buffer & Spring
The buffer system lives in the stock and absorbs the rearward energy of each shot, then returns the bolt carrier forward. Buffer weight (carbine, H, H2, H3) is matched to your gas system length and calibre — heavier buffers slow the bolt velocity, which matters if you're running a suppressor or an adjustable gas block. Carbine buffers pair with carbine stocks; rifle-length buffers pair with fixed A2-style stocks.
Stock
The stock is what goes against your shoulder. Mil-spec tube stocks (6-position collapsible) are the default. Fixed stocks (A2, Magpul PRS) give a more stable cheek weld for precision builds. If you're building a pistol configuration, you'll use a pistol brace instead — and your buffer tube becomes a pistol buffer tube, which changes diameter.
Upper Receiver
The upper receiver is the top half of the rifle. It houses the barrel extension, accepts the handguard, and is where the bolt carrier group sits during cycling. Flattop uppers with a Picatinny rail are standard. Like lowers, forged 7075-T6 is the common material. Stripped uppers require you to install the barrel and BCG; complete uppers come assembled. Buying a matched upper/lower set from the same manufacturer avoids any fit issues.
Barrel
The barrel is where calibre, gas system length, twist rate, and profile all intersect — and it's arguably the most build-defining part you'll choose.
Gas system length (pistol, carbine, mid-length, rifle) should match your barrel length. Mid-length gas on a 16" barrel is a popular, softer-shooting combination. Twist rate (1:7, 1:8, 1:9) determines what bullet weights stabilise well — 1:8 is the most versatile. Profile (light, government, SOCOM, heavy) affects weight and heat dissipation.
Handguard / Rail
The handguard protects your support hand from a hot barrel and gives you somewhere to mount accessories. M-LOK and KeyMod are the two attachment systems; M-LOK has largely won. Free-floating handguards (attached only at the upper receiver) improve accuracy by removing barrel pressure. Drop-in handguards are easier to install and cheaper. Length should match or complement your gas system without covering the gas block.
Gas Block & Gas Tube
The gas block sits on the barrel at the gas port and directs propellant gas back into the gas tube to cycle the action. Low-profile gas blocks sit under the handguard and accept sling mounts and adjustable gas valves. Adjustable gas blocks (Superlative Arms, SLR Rifleworks) let you tune gas flow — essential for suppressed use or very light/heavy loads.
Bolt Carrier Group (BCG)
The BCG is the heart of the cycling mechanism: bolt, carrier, cam pin, firing pin. It strips a round from the magazine, chambers it, fires it, extracts the spent case, and resets. Quality matters here — look for properly staked gas keys, properly cut/gauged bolt heads, and good carrier finish (nitride/phosphate/nickel boron). This is not the place to save $20.
Charging Handle
The charging handle is how you manually operate the bolt — chamber a round, clear a malfunction, lock the bolt back. Mil-spec handles work fine. Extended latch handles (BCM Gunfighter, Radian Raptor) make one-handed manipulation easier, especially with optics or suppressors that push your head position back. A minor upgrade with a real quality-of-life difference.
Muzzle Device
The muzzle device threads onto the barrel's muzzle end. Flash hiders reduce the signature of each shot. Compensators reduce muzzle rise. Muzzle brakes reduce felt recoil more aggressively but are loud. Suppressors attach over the muzzle (either direct-thread or via a muzzle device mount). Thread pitch must match your barrel — 1/2×28 is standard for 5.56/.223; 5/8×24 is standard for .308/6.5 Creedmoor.
That's every major slot in a standard AR-15 build. If you're ready to start speccing parts, the Battl Builder walks you through each slot with real retailer pricing.