The Ultimate AR Platform Resource: The Complete Engineering Guide

Phase 0: 80% Lowers & Build Roadmap

If this is your first time building a firearm, you need to understand where the 80% Lower fits into the ecosystem. The lower receiver is the only part of the AR-15 that is regulated as a "firearm." Everything else—barrel, bolt, stock—are just unrestricted parts.

Who is this guide for?

This guide acts as a roadmap for two types of builders:

1. The Home Machinist: You purchased an 80% lower and need to understand the physics of the platform to build a safe, reliable rifle.
2. The Custom Builder: You want to hand-select every pin and spring to create a rifle that outperforms factory standards.

Phase I: The Receiver Foundation

The lower receiver is the structural anchor of the entire platform. While many assume all lowers are equal, the metallurgy and finishing process dictate the lifespan and corrosion resistance of your rifle.

1. Metallurgy: 7075-T6 vs 6061-T6

Not all aluminum is created equal. The "T6" refers to the heat treatment process (Solution Heat Treated and Artificially Aged), which gives the metal its hardness.

2. Manufacturing Methods: Forged vs Billet

• Forged (The Gold Standard): Aluminum is hammered into shape under immense pressure. This compresses the grain structure to follow the contours of the part, offering maximum impact resistance.
• Billet (The Custom Choice): CNC machined from a solid block. The grain structure is linear. Modern billet receivers are over-engineered with thicker walls to compensate for grain structure, offering unique aesthetics.

3. Finishing: Protecting the Metal

If you are building on a raw 80% receiver, understanding protection is vital. Aluminum does not rust, but it oxidizes (turns chalky white).

Phase II: The 80% Lower Concept

The "80% Lower" is a popular entry point for home builders who wish to understand the internal mechanics of their firearm intimately. It allows you to participate in the manufacturing process.

1. The Design Checklist

When selecting an 80% lower, look for these features to ensure compatibility:

• Broached Magwell: Ensure the magazine well is cut to Mil-Spec tolerances. Cheap lowers often have tight magwells that won't drop magazines freely.
• Rear Takedown Pocket: Some 80% lowers come with the rear pocket (where the upper receiver lug goes) already milled. This saves you a difficult machining step.
• Safety Selector Stops: Verify the markings/stops match your intended safety (90 degree vs 45 degree throw).

2. Jigs and Tooling: How it Works

To complete an 80% lower, builders utilize a Jig. A jig is a fixture that clamps the receiver in place and provides a template.

• Router Jig (Recommended): Uses a handheld woodworking router. The jig guides the router to remove material in layers. This produces the cleanest, factory-like finish.
• Drill Press Jig: Uses a drill press to bore vertical holes, then connects them. This requires more manual cleanup with files and produces a rougher internal finish.

Phase III: The Armorer's Workbench

You cannot build a precision rifle with a pair of pliers. Having the right tools prevents scratched receivers, broken parts, and frustration.

Phase IV: Barrels & Muzzle Devices

The barrel is the heart of the rifle. Its profile, lining, and muzzle device determine accuracy, handling, and recoil impulse.

1. Twist Rate: Stabilizing the Projectile

2. Barrel Materials

• 4150 CMV: The Mil-Spec steel. Highly durable, handles heat well. Usually Parkerized or Nitrided.
• 416R Stainless: Softer steel designed for precision machining. More accurate, but shorter lifespan than 4150.

3. Muzzle Devices: The Triad

• Flash Hider (A2): Breaks up the gas expanding from the barrel to minimize visual signature. Standard for duty.
• Muzzle Brake: Redirects gas sideways/backwards to pull the rifle forward, cancelling recoil. Warning: Very loud for people standing next to you.
• Compensator: Vents gas upwards to push the barrel down, reducing muzzle rise during rapid fire.

Phase V: Handguards & Optics

1. Attachment Standards

• M-LOK: The USSOCOM standard. Slots allow T-nuts to lock in. Lightweight and smooth.
• Quad Rail (1913 Picatinny): The classic "cheese grater." Heavy, but allows instant mounting anywhere. Very rigid.

2. Optic Mounting Physics

Bridging the Gap: Never mount your optic rings so that they span across the receiver and the handguard. The handguard flexes under heat and pressure; the receiver does not. Bridging causes your zero to shift.

Phase VI: Gas System Physics

The number one reason for a home-built AR failing is the gas system. The AR-15 operates on "Direct Impingement"—gas is siphoned from the barrel to blow the bolt carrier back.

1. Dwell Time

Dwell time is the amount of time the bullet spends in the barrel after passing the gas port but before exiting the muzzle. This pressurized time is what cycles the bolt.
Too little dwell time (short barrel, long gas system) = Short Stroking.
Too much dwell time (long barrel, short gas system) = Harsh recoil and wear.

2. Gas Block Installation: Dimpling vs. Pinning

A gas block that shifts cuts off gas flow, turning your semi-auto into a single-shot rifle.

• Set Screw (Dimpled): Using a jig to drill a small divot in the barrel directly under the set screw. The screw locks into this divot. Highly recommended for home builders.
• Pinned (Bombproof): Drilling a taper pin through the side of the block and the barrel curve. This is the Mil-Spec requirement for duty rifles.

3. The Buffer Weight Balancing Act

Phase VII: The BCG & Lubrication

1. The Lubrication Map

AR-15s like to run "wet," but putting oil in the wrong place attracts carbon.

• Lube HEAVY (Wet): The 4 exterior rails of the Bolt Carrier (where it touches the receiver) and the Cam Pin.
• Lube LIGHT (Sheen): The Bolt Lugs and the Buffer Spring (to reduce noise).
• Run DRY: The Gas Key, The Gas Tube, and the Face of the Bolt. Oil here turns into carbon paste.

2. Coatings Comparison

Phase VIII: Buffer & Recoil Systems

1. Buffer Weights Explained

• Carbine (3.0 oz): Standard 3 steel weights. Use for 16" Mid-Length or weak ammo.
• H1 (3.8 oz): 1 Tungsten, 2 Steel. General Purpose. Good for 14.5" - 16" Carbine gas.
• H2 (4.6 oz): 2 Tungsten, 1 Steel. Best for MK18s (10.3") or Suppressed shooting. Delays unlocking.
• H3 (5.4 oz): 3 Tungsten. Used for short suppressed rifles or to tame violent actions.

2. PCC (9mm) Blowback Physics

PCCs do not use gas tubes. They rely on the sheer mass of the bolt to keep the chamber closed. The Golden Rule: Ensure your BCG + Buffer weight totals 22oz - 24oz. Anything lighter may result in "out of battery" detonations.

Phase IX: Lower Parts & Triggers

1. Trigger Geometry

• Single Stage: No "take-up." You apply pressure and the wall breaks instantly (like a glass rod). Preferred for fast, tactical shooting.
• Two-Stage: There is a light, smooth "take-up" phase before you hit a distinct wall. Applying slightly more pressure breaks the shot. Preferred for precision/distance.

2. Magazines: The Source of 90% of Failures

Phase X: Assembly & Torque

This section is your cheat sheet. Print this out. Building without torque specs is dangerous.

1. Critical Torque Specifications

2. What is "Staking"?

Staking is the process of using a center punch to displace metal from one part (like the receiver end plate) into the locking notches of another part (the castle nut). This creates a physical barrier that prevents the nut from backing off under vibration. It is a mandatory step for a reliable build.

Phase XI: Troubleshooting & Zeroing

1. The FTF/FTE Matrix

• Short Stroking (Bolt doesn't lock back): Under-gassed (misaligned block) or Buffer too heavy.
• Stovepiping (Case gets stuck in ejection port): Weak extractor spring or weak ejector.
• Failure to Feed: Magazine not seated, dirty feed ramps, or buffer spring too weak.

2. Zeroing: The 36 Yard vs 50 Yard Zero

Because the sight sits ~2.6" above the barrel, the bullet must arc up to meet your line of sight.

• 50/200 Zero: The most common. Bullet crosses line of sight at 50 yards and again at 200 yards. Very flat trajectory.
• 36 Yard Zero: Popularized by the USMC. Allows for a "Point Blank Zero" out to 300 yards with minimal hold-over.

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