Civilian IAR Build Guide 2026: AR-15 Automatic Rifle Setup (Block 2, Hopsaw, Belt-Fed)

In 2010, the USMC adopted the M27 Infantry Automatic Rifle to replace the M249 SAW in the squad automatic rifleman role. The decision was controversial: a magazine-fed rifle replacing a belt-fed machine gun. But the Marines had data showing that accurate, sustained semi-automatic and burst fire from a heavy-barreled rifle suppressed enemy positions more effectively than poorly aimed belt-fed fire, while cutting the weight each Marine carried by over 10 pounds.

The civilian IAR applies the same logic. With forced reset selectors providing near-cyclic fire rates and heavy barrel profiles handling the thermal load, an AR-15 can fill a suppressive fire role that was previously exclusive to NFA weapons. This is not about replicating a belt-fed machine gun. As Hop puts it: "The civilian fire support weapon is a lot closer to the US Marine Corps' M27 IAR than it is to an M249 SAW." It is a companion to your SPR or general-purpose rifle, giving a team the ability to lay suppressive fire during movement.

The Super Safety Guide covers FRS mechanics, installation, and tuning in detail. This guide assumes you understand how forced reset selectors work and focuses on the platform itself: which barrel, which rail, which buffer system, and why.

Every component in the IAR build exists to manage a single variable: heat. Under sustained fire, the chamber temperature climbs until it reaches the cook-off threshold, approximately 400-500F, at which point the propellant in a chambered round can auto-ignite without a trigger press. That is not a malfunction. It is a catastrophic safety hazard. If you have a round chambered and the barrel is at cook-off temperature, it will fire on its own.

The failure progression under extreme heat is predictable. The gas tube fails first because it is designed as a sacrificial component, thin-walled steel that sags and eventually splits before the barrel itself fails. This is by design: a gas tube failure stops the rifle from cycling, which is preferable to barrel failure. After the gas tube, the barrel droops (literally bends under its own weight at extreme temperatures), and eventually the barrel itself can fail catastrophically.

What Slows Heat Buildup

Barrel profile is the single biggest factor. A heavier barrel has more metal to absorb heat before reaching critical temperatures. An HBAR profile can absorb roughly 70% more thermal energy than a Pencil profile before reaching the same chamber temperature. The SOCOM profile splits the difference, offering 35% more thermal capacity than Government profile at a manageable weight penalty.

Gas system length matters because longer gas systems distribute heat more evenly along the barrel. A mid-length gas system runs cooler than carbine length at the same barrel length because the gas port is further from the chamber. Rifle-length gas is even better but requires 18"+ barrels. For a 14.5" IAR, mid-length gas is the right choice. For a 16" Hopsaw, mid-length is standard. See our gas system tuning guide for detailed buffer and gas block setup.

Handguard design affects heat dissipation rate. This is why we recommend the RIS II quad rail for the Block 2 IAR: the aluminum quad rail has significantly more surface area and thermal mass than an M-LOK handguard. It acts as a passive heat sink, drawing thermal energy from the barrel nut and barrel extension outward into the rail. The trade-off is weight, but in the IAR role you are already accepting a heavier rifle for the suppressive fire capability.

Use a pinned gas block, not adjustable. Adjustable gas blocks are a liability under sustained fire. Carbon buildup clogs the adjustment mechanism, detents seize under heat, and you are left with an unreliable single point of failure. A quality low-profile gas block, drilled and pinned to the barrel by a gunsmith, is permanently secured, maintenance-free, and will never drift or fail. Tune your gas system through buffer weightBuffer Weight[Gas System]The mass of the buffer (Carbine, H1, H2, H3) inside the receiver extension. Heavier buffers delay the unlocking of the bolt, helping manage gas pressure and recoil. and spring rate instead.

The Block 2 M4 with a SOCOM-profile barrel and Daniel Defense RIS II quad rail is the foundation of the IAR build we recommend for most shooters. It is not the cheapest option. It is not the most thermally capable. It is the one that balances heat management, weight, accuracy, and parts availability into a rifle you can actually carry and fight with.

The RIS II quad rail is a deliberate choice over modern M-LOK alternatives. The four aluminum Picatinny rail segments add thermal mass that acts as a passive heat sink, pulling energy from the barrel nut outward. It also gives you unrestricted bipod mounting on the bottom rail and direct-attach capability for lights, lasers, and other accessories without adapters. Weight penalty is roughly 3 oz over a comparable M-LOK rail. For the IAR role, that is a worthwhile trade.

If you already have a Block II clone build, you are halfway there. Add the VLTOR A5 buffer system and drop in your FRTFRT[Components]Forced Reset Trigger. A trigger design where the firearm's cycling action mechanically forces the trigger forward to reset, rather than relying solely on the trigger spring. Enables faster follow-up shots than standard semi-automatic triggers. Federal legality restored in 2025, but banned in 15 states. of choice. The DD M4A1 SOCOM upper (available through Brownells) already has the RIS II quad rail and SOCOM barrel you need.

The Hopsaw concept, named by Hop, strips the IAR down to its essential variable: barrel thermal mass. Our take builds it as a parts list: a Geissele CHF barrel (14.5" or 16") on a mid-length gas system, Aero M4E1 stripped upper, Midwest Industries Combat Rail, and a standard carbine buffer tube running an H2 buffer and Sprinco Blue spring. No exotic heat sinks. No piston conversion. Just a heavy barrel doing what heavy barrels do.

The counterintuitive truth is that the Hopsaw is thermally superior to the more expensive Block 2 build. The HBAR profile has roughly 30% more thermal mass than the SOCOM profile, which means more rounds before reaching cook-off temperatures. The trade-off is weight and handling. A 16" HBAR is noticeably front-heavy, which makes it less agile for transitions and movement. But in the prone-supported IAR role, that weight is an advantage, not a penalty.

The Hopsaw proves a point Hop makes in his video: avoid "cargo cult" thinking. Adding a 20" barrel, a carry handle, and a tripod does not make your AR-15 a machine gun. It makes it unmanageably heavy with little performance gain. The Hopsaw keeps things practical: a rifle that handles sustained fire well and can still be carried, maneuvered, and employed as a standard rifle when the IAR role is not needed.

Purpose-built IAR uppers from manufacturers like Red Right Hand take heat management beyond barrel profile selection. The Red Right Hand RECCE IAR features machined aluminum heat sinks press-fit onto the barrel that actively draw thermal energy away from the chamber. A coiled heat sink on the gas tube prevents the most common failure mode under sustained fire. And a Superlative Arms short-stroke piston conversion reduces the volume of hot gas entering the receiver.

The V2 uses a Shaw-manufactured .850" BAR-profile barrel (14.5", 4150 CMV, chrome-lined, 1:7 twist) with a Midwest Industries 12.65" Combat Rail and Superlative Arms short-stroke piston system with adjustable gas. Despite the heat sinks and piston conversion, the complete upper weighs only 4-5 oz more than the budget Hopsaw upper.

This tier makes sense for shooters who run their IAR hard and often, who need maximum round count before cooling, and who can justify the $1,510 upper-only price tag. For most shooters, the Block 2 build delivers 90% of the capability at 40% of the cost.

For shooters who want to go beyond the IAR concept into true SAW territory, the FightLite MCR and its successor the DFX are belt-fed upper receivers that mount on any standard AR-15 lower. The MCR feeds from disintegrating M27 belts or standard STANAG magazines, giving you 200+ rounds on tap without magazine changes. With an FRS installed, Brass Facts demonstrated 700+ rounds of sustained fire in testing.

The belt-fed path is expensive ($3,000+ for the upper alone), heavy (adds ~4 lbs over a standard upper), and logistically demanding (belted ammo, feed trays, and the MG34 heavy spring required for reliable FRS cycling). It is also arguably more reliable than the M249 SAW it emulates, since it uses the proven AR-15 bolt and barrel system rather than the M249's open-bolt mechanism.

We covered the latest FightLite DFX upper at SHOT Show 2026. For most shooters, the magazine-fed IAR builds above are the more practical choice. The belt-fed path is for dedicated fire support specialists who train regularly with high-volume fire.

The M27 IAR is fielded with the EOTech holographic sight and G33 3x magnifier. This combination works because the IAR role requires two different things from its optic: fast, wide-field-of-view target acquisition during suppressive fire (1x holographic), and positive target identification at extended range between suppression tasks (3x magnifier). An LPVOLPVO[Optics]Low Power Variable Optic. A scope that goes from 1x (no magnification) to 6x, 8x, or 10x. A versatile choice for close quarters to mid-range engagements. can do both, but the EOTech transitions faster and the 68 MOAMOA[Optics]Minute of Angle. At 100 yards, 1 MOA ≈ 1 inch. Used to measure rifle accuracy (e.g., '1 MOA rifle') and adjustment clicks on scopes. ring is purpose-built for rapid engagement.

The ELCAN SpecterDR 1-4x is the other serious IAR optic. It switches between true 1x and true 4x with a throw lever, no zoom distortion, no loss of eye reliefEye Relief[Optics]Distance from your eye to the optic's rear lens where you get a full sight picture. Critical for comfort, safety (avoiding scope eye), and proper use with magnified optics.. The 4x gives you more reach than the G33's 3x, and the integrated design eliminates the two-item mounting problem. The tradeoff is weight (~24 oz) and price (~$2,500). If your IAR budget allows it, the SpecterDR is a direct upgrade over the EOTech + G33 combination.

Fixed prism optics are the budget play. The Primary Arms SLx 3x MicroPrism gives you a fixed 3x with an etched ACSS reticle for $350. No battery required for reticle visibility, works perfectly for shooters with astigmatism, and the compact footprint keeps weight down. You lose the 1x CQB capability of a holographic, but the wide eyebox and 3x magnification cover the IAR's primary engagement range of 100-300 meters. For a complete breakdown of optic categories, see our optic selection guide.

The IAR role consumes ammunition at a rate that changes your entire loadout calculation. At ~450 RPM with an FRS, a 30-round magazine empties in roughly 4 seconds. A D-60 drum lasts about 8 seconds. A realistic suppressive fire string, covering a team element's movement to the next position, requires 60-120 rounds depending on distance and duration.

A practical IAR combat load is two Magpul D-60 drums and four 30-round PMAGs, totaling 240 rounds. The D-60s go in dedicated drum pouches (Tactical Tailor makes one), and the PMAGs ride in standard triple-mag shingles. Brass Facts recommends the Agilite Reaper chest rig with expansion wings for the increased weight.

SureFire 60-round coffin magazines are an alternative to the D-60 for shooters who prefer a box magazine form factor. They fit standard mag pouches better than drums but are heavier per round. Magpul 40-round PMAGs split the difference between capacity and profile for follow-up magazines. For more on magazine selection, see our AR-15 magazine guide.

Suppressors and the IAR role are in tension. A suppressor increases backpressure, which accelerates heat buildup in the receiver and bolt carrier group. It also traps heat at the muzzle end of the barrel, which is already the hottest section during sustained fire. The result is a shorter timeline to cook-off and increased gas blowback into the shooter's face.

If you run a suppressor on your IAR, use a flow-through design like the Huxwrx Flow 556k rather than a traditional baffled can. Flow-through suppressors produce virtually no increase in bolt carrier velocity, which means you do not need to retune your buffer system or adjust your gas block when mounting the can. A conventional baffled suppressor increases backpressure significantly, which accelerates carrier speed, increases heat in the receiver, and worsens gas blowback into the shooter's face. With a flow-through design, your IAR runs the same suppressed as unsuppressed. See our suppressor compatibility guide for detailed tuning recommendations.