Description
The 13.9″ SOTM4 MOD-0 Forged Monobloc® Complete Upper pairs an HM Defense Monobloc® barrel with SOT’s MOD-0 forged upper receiver. The gas block is milled straight into the metal of the barrel blank during manufacturing, so there is no separate block to clamp, pin or align. The barrel is thermal fitted to the receiver with no retaining compound or bedding.
It ships as a complete upper with the SOT Enhanced HMB™ bolt carrier group, a Breek Arms WARHAMMER Mod2 ambidextrous charging handle and a 13″ SOT Wedge-Lock free-float M-LOK handguard, ready to install on any Mil-Spec AR-15 lower receiver.
The chrome-lined bore adds erosion life and corrosion protection. Chrome-lined Monobloc barrels are produced in limited numbers.
Every SOT upper is built by our team to a strict, documented set of standards and processes. Bombproof by Design™.
Barrel
- 13.9″ HM Defense Monobloc® barrel, 5.56×45mm NATO chamber, .223 Remington compatible
- 4150 chrome-moly steel, chrome-lined bore
- Chrome-lined bore for added erosion life and corrosion protection; offered in limited numbers
- Balanced barrel taper/profile
- 1:8 twist
- 1/2×28 muzzle thread
- Mid-length gas system
- Gas block milled straight into the metal of the barrel blank during manufacturing, so there is no separate block to clamp, pin or align
- Threaded barrel extension
- Black Phosphate finish
MOD-0 Forged Upper Receiver
- Forged from 7075-T6 aluminum
- Precision machined to Mil-Spec M16/M4 specifications, with tighter tolerances than a standard forged upper
- Barrel extension bore machined to .9980″ inside diameter, slightly undersized for a tight fit; the barrel is thermal fitted into it with no retaining compound or bedding
- Lugs machined to .4960″
- .250″ takedown pin holes
- M4 feed ramps
- Forward assist, phosphate finish
- Laser-engraved T-marks on the top rail
- Type III hard-coat anodized, black, with dry film lubrication
- Strict Mil-Spec dimensions for universal cross-brand compatibility
- Ejection port cover laser engraved with the caliber on both the interior and exterior, so the shooter can confirm caliber at a glance
Bolt Carrier Group
- SOT Enhanced HM Defense HMB™ bolt carrier group, Black Nitride (Melonite®)
- Bolt: 9310 steel, CNC machined with a blind cam pin socket instead of a through-hole
- Carrier: 9310 steel, Mil-Spec M16 full-auto profile
- Cam pin: 4340 hardened steel, custom radiused design to fit the socket
- Weight: approximately 12.72 oz
- SOT arrow-indexed cam pin, which ensures the cam pin is always installed in the same direction for better wear and cam pin life
- Index-marked gas key
- Platform: AR-15 / M16, compatible with standard Mil-Spec upper receivers
- The HMB™ bolt group is also compatible with .300 AAC Blackout, 6mm MAX and 350 Legend. This upper is chambered in 5.56×45mm NATO and is .223 Remington compatible.
Charging Handle
- Breek Arms WARHAMMER Mod2 AR-15 ambidextrous charging handle, black
- Oversize ambidextrous release latches on both sides with textured grooves for gloved hands, set wide and back to clear large optics
- One-handed operation from either side
- Gas relief cuts on the top and bottom of the stem vent gas out the side instead of toward the shooter’s face, and a raised rear shelf covers the back of the upper receiver; both help most when shooting suppressed
- 7075-T6 aluminum handle and latches, Type III hard-coat anodized
- Just under 1 oz; 2.78″ across the handles
- Made in the USA by Breek Arms
Handguard
- 13″ SOT Wedge-Lock free-float M-LOK handguard, manufactured to SOT specification
- 6061-T6 aluminum, hard-coat anodized
- Full-length top Picatinny rail with M-LOK on the other seven sides
- Wedge-Lock twin-lug wedge fastening to a steel HD barrel nut, so the rail will not flex under bipod load
Muzzle
- A2 Extended Flash Hider, Black Cerakote®, standard
- Optional add-on: 4-port hardened flash hider, Black Cerakote®, $40.00
- Optional add-on: Faxon removable MuzzLok® muzzle brake, $60.00
- Optional add-on: Faxon MuzzLok® Plan B muzzle brake, $80.00 (17-4 PH stainless steel, nitride finish, timed with a left-hand nut; its Plan B interface accepts a Plan B-to-HUB adapter for hub-mount suppressors, adapter sold separately)
- Other muzzle devices and suppressor mounts are available; contact us directly to discuss options.
- Configuration shown in the photos: optional 4-port hardened flash hider, Black Cerakote®. This upper ships with the standard Black Cerakote® Extended A2 Flash Hider.
Sights
- None standard: the full-length top Picatinny rail is ready for the optic of your choice
- Optional add-on: Magpul MBUS 3 front & rear flip-up sight set (MAG1166-MAG1167), black, $75.00: impact-resistant polymer, spring-loaded with an ambidextrous push-button, tool-less front elevation, rapid-select rear aperture, 0.44″ high folded, standard AR height deployed, Picatinny mount, made in the USA
What Ships & What Fits
- Ships as a complete upper receiver group, including bolt carrier group and charging handle
- Fits any Mil-Spec AR-15 lower receiver
- Works with carbine, VLTOR A5 and rifle buffer systems
Ships direct. An upper receiver is not a firearm, so no FFL transfer, background check or dealer fee is required in most states. Some state and local exceptions apply.
Barrel length & your lower. This upper has a 13.9″ barrel, so installing it on a lower receiver with a stock makes a short-barreled rifle (SBR). As of October 9, 2026, ATF no longer requires NFA registration, approval or a tax stamp to make or own an SBR. This is a federal enforcement policy, not a change to the law, and it could change. Federal rules still require ATF Form 5320.20 approval before you take an SBR across state lines.
Many states and localities regulate or prohibit the ownership or possession of short-barreled rifles. Know your state and local laws before installing this upper on a stocked lower; compliance is the purchaser’s responsibility.
Inspect this upper up close: zoom in on both sides →
Shipping: a complete upper is not a firearm and ships direct to you. We do not ship upper receivers to CA, CT, DE, DC, HI, IL, MA, NJ, NY, RI or WA, or to Denver, CO or Cook County/Chicago, IL. Monobloc® uppers are marked and marketed as HBAR and can ship to Maryland.
WARNING: This product can expose you to chemicals including lead, which is known to the State of California to cause cancer and birth defects or other reproductive harm. For more information go to www.P65Warnings.ca.gov.
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