The Monobloc® rifle on a thermal-fitted forged receiver set
16″ rifle · 14.5″, 13.9″ and 12.5″ short barrel rifles · SOTM4MOD1 A5 complete lower
Stronger metal around the joints, and fewer joints that can move.
SHOWN: 16″ SOTM4MBA5 Forged Rifle, 15″ SOT Wedge-Lock M-LOK handguard, FDE anodized, with FDE B5 Systems Bravo furniture. Below: the rest of the series.
The step down in price, not in the parts that keep it running.
The SOTM4A5 Forged Series is the Monobloc® rifle from our billet flagship, built on a forged receiver set. The barrel, the gas system, the bolt carrier group and the recoil system are the same. The upper and lower are forged 7075, and the barrel is thermal fitted to the forged MOD-0 upper. It costs less because a forging takes less machine time than billet, not because anything that keeps the rifle running was taken out. From $1,450.00.
Strength from the grain
Pressed to shape, so the grain follows the receiver: stronger and tougher than billet of the same alloy.
A barrel joint that holds itself
An interference fit between receiver and barrel extension, with no compound and no bedding.
Failure points removed
A gas block that can't loosen, a bolt that keeps its metal, a gas key you can read, a recoil system that stays put.
Stronger where an AR takes its knocks
Forging presses the 7075 aluminum into shape instead of cutting the receiver out of a solid block, so the grain of the metal follows the shape of the part. That makes a forged receiver stronger than a billet receiver of the same alloy, and tougher against impact and hard use.
Fits like every Mil-Spec receiver
Both forged receivers are held to Mil-Spec dimensions, so they work with Mil-Spec parts across brands. The SOTM4MOD1 A5 complete lower, sold on its own, pairs with any Mil-Spec AR-15 upper.
Priced for the machine time it saves
A forging comes out close to its finished shape, so it needs far less machining than billet. That is where the price difference comes from. Billet buys tighter tolerances between upper and lower and room for features like SOT's profile and an ambidextrous bolt release. We build both.
On most AR rifles the barrel is held in the upper by the barrel nut: the extension slides into the receiver and the nut clamps it. On the MOD-0 forged upper we fit the barrel thermally, so the receiver itself grips the barrel extension.
How it is done
The upper receiver is expanded with heat and the barrel extension chilled, so the two seat without force. As they return to the same temperature, the receiver closes on the extension in an interference fit.
What it gives you
A barrel joint that does not depend on the barrel nut's clamping force alone to keep the barrel square to the receiver. No galling on assembly, no retaining compound, no bedding.
Why it suits the forged upper
MOD-0 is how Second of Ten designates a thermal-fitted receiver: it names how the barrel is held. On this series it is the forged upper, so a forged receiver set carries the same disciplined barrel joint as our other MOD-0 builds.
Thermal fitting secures the joint. It is not a group-size promise: accuracy comes from the barrel, the ammunition and the shooter.
Bombproof by Design™ is how we choose parts: find the documented failure points on an AR, and remove them at the component instead of managing them with maintenance. The forged series carries the same four answers as our billet flagship.
The gas block that cannot come loose
On an HM Defense Monobloc® the gas block is milled straight into the metal of the barrel blank during manufacturing. There is no separate block to clamp, pin or set-screw, nothing to shift, and no port to misalign. The port is sized at manufacture for mid-length gas, even at 12.5 inches.
The bolt that keeps its metal
The cam pin hole is the documented weak point of the AR bolt. The HMB™ bolt replaces the through-hole with a socket, so material is retained where a standard bolt removes it. Its radiused cam pin carries the SOT arrow index, so it always goes back in the same way, for even wear and longer cam pin life.
A gas key you can check at a glance
The gas key is staked, and a single line is laser engraved across the key and both screw heads. A screw that has started to turn shows as a stepped line during ordinary cleaning, long before it can leak gas and short-stroke the rifle. If the line is broken, contact us before firing.
A recoil system that stays put
The SOT A5 nine-position buffer tube runs an A5H2 buffer and a rifle-length action spring: the same energy over a longer runway, for a lower peak. The Forward Controls Design CN5F extended castle nut is staked, with Vibra-TITE VC-3 on the threads, so the tube cannot back out.
The trade-off we accept: a Monobloc® gas port is sized at manufacture, so it cannot take an adjustable gas block.
| Model | Barrel | Handguard | Length, collapsed / extended | Weight, unloaded | Price |
|---|---|---|---|---|---|
| 16″ SOTM4MBA5 Forged Rifle Rifle | 16″ HM Defense Monobloc® | 15″ M-LOK · 13″ M-LOK · 13″ Quad Rail (FDE quad rail coming soon) FDE or black | 34″ / 37″ | 6 lb 15 oz / 6 lb 11 oz / 7 lb 11 oz | $1,450.00 – $1,500.00 |
| 14.5″ SOTM4MBA5 Forged Short Barrel Rifle, Chrome-Lined, 13″ M-LOK Short barrel rifle | 14.5″ HM Defense Monobloc®, chrome-lined | 13″ M-LOK Black | 32.5″ / 36″ | 6 lb 8 oz | Price to be confirmed |
| 13.9″ SOTM4MBA5 Forged Short Barrel Rifle, Chrome-Lined, 13″ M-LOK Short barrel rifle | 13.9″ HM Defense Monobloc®, chrome-lined | 13″ M-LOK Black | 31.9″ / 35.4″ | 6 lb 7 oz | Price to be confirmed |
| 12.5″ SOTM4MBA5 Forged Short Barrel Rifle, 11.7″ M-LOK Short barrel rifle | 12.5″ HM Defense Monobloc® | 11.7″ M-LOK Black | 31.5″ / 34″ | 6 lb 6 oz | $1,450.00 |
| SOTM4MOD1 A5 Forged Complete Lower Receiver Complete lower receiver | The forged lower the series is built on, sold complete: Schmid two-stage DLC trigger, Geissele Ambi Posi-Snap and Maritime, SOT A5 nine-position tube, A5H2, B5 Systems Bravo stock and grip in FDE or black. Upper not included. | $625.00 FDE or black | |||
Every rifle: 5.56×45mm NATO (.223 Remington compatible), 4150 chrome-moly, match-grade, triple-honed, button-rifled, 1:8, mid-length gas · SOT Enhanced HMB™ group in black nitride · one 30-round magazine: Lancer L5AWM® with FDE builds, to match; Magpul PMAG® 30 GEN M3™, SOT crest engraved, with black builds · Breek Arms WARHAMMER charging handle · forward assist · Schmid Inside Elite two-stage trigger · Geissele Ambi Posi-Snap and Maritime · B5 Systems Bravo stock and Type 23 grip.
| SOTM4A5 Forged Series | SOTM4A5 billet flagship | |
|---|---|---|
| Strength | Forged: grain follows the shape of the receiver. The stronger, more impact-resistant of the two | Billet: machined from a solid block |
| Fit | Mil-Spec dimensions, fits Mil-Spec parts across brands | Far tighter tolerances between upper and lower |
| Barrel joint | Monobloc® barrel thermal fitted to the forged MOD-0 upper | Monobloc® barrel in the SOT billet upper |
| Barrel, gas, bolt | The same on both: HM Defense Monobloc®, mid-length gas milled into the barrel blank, SOT Enhanced HMB™ group | |
| Bolt release | Left side (Geissele Maritime) | Ambidextrous, with a right-side release |
| Standard trigger | Schmid Inside Elite two-stage, DLC; G2S® or LaRue MBT-2S +$100 | LaRue MBT-2S or Geissele G2S® |
| Price | From $1,450.00 | See the flagship series |
| Option | Choice | Price |
|---|---|---|
| Trigger | Schmid Inside Elite 2-Stage, DLC, curved bow (standard) | Included |
| Geissele Automatics G2S® two-stage, installed | +$100.00 | |
| LaRue Tactical MBT-2S two-stage, curved or straight bow, installed | +$100.00 | |
| Muzzle device | A2 Extended Flash Hider, Black Cerakote® (standard) | Included |
| 4-port hardened flash hider, Black Cerakote® | +$40.00 | |
| Faxon MuzzLok® Gunner 3-port 5.56 steel muzzle brake | +$60.00 | |
| Faxon MuzzLok® stainless steel Plan B muzzle brake | +$75.00 | |
| Bolt carrier group | SOT Enhanced HMB™, Black Nitride (standard) | Included |
| Young Manufacturing M16 Chrome BCG with HMB™ bolt (supplied as Young build it, without SOT's enhancements) | +$149.00 | |
| Microbest Super-Stake C158 all-chrome BCG, heavy-duty stake, with chrome HMB™ bolt | +$149.00 | |
| Sights | Magpul MBUS 3 front & rear flip-up set, black (MAG1166-MAG1167) | +$75.00 |
The complete lower offers the trigger choice only. Other muzzle devices and suppressor mounts on inquiry.
Where the series can ship
Not available in California, Connecticut, Delaware, Illinois, Maryland, Massachusetts, New Jersey, New York, Rhode Island, Washington or Washington, D.C. The short barrel rifles are also not available in Hawaii. In Colorado and Vermont, and wherever magazine-capacity limits apply, rifles ship with compliant magazines instead of the 30-round magazine. Laws change: please confirm with your local FFL before ordering.
Under ATF's October 9, 2026 guidance, a short barrel rifle no longer needs NFA registration or a tax stamp. State and local laws still apply, and some states prohibit short barrel rifles. Confirm with your FFL before ordering.
Photography © Second of Ten LLC. MONOBLOC® and HMB™ are marks of HM Defense, LLC, used under license. Bombproof by Design™ is a mark of Second of Ten LLC. Cerakote® is a mark of NIC Industries; G2S®, Posi-Snap and Maritime of Geissele Automatics; MBT-2S of LaRue Tactical; Inside Elite of Schmid Inside; WARHAMMER of Breek Arms; CN5F of Forward Controls Design; Bravo and Type 23 of B5 Systems; L5AWM® of Lancer Systems; PMAG®, GEN M3™ and MBUS® of Magpul Industries; MuzzLok® of Faxon Firearms; Vibra-TITE of ND Industries; M-LOK® of Magpul Industries. Marks are the property of their owners; their appearance indicates that Second of Ten incorporates the component, not sponsorship, affiliation or endorsement. Prices in USD, subject to change; the series page shows current prices and availability. Every Second of Ten firearm is semi-automatic. Second of Ten LLC · Stuart, Florida · FFL 07 / SOT Class 02.
| Version | Date | Change |
|---|---|---|
| 1.0 | 2 NOV 2026 | First issue |
| 1.1 | 2 NOV 2026 | Complete lower priced at $625.00 (FDE or black); magazine per finish added (Lancer L5AWM® with FDE, Magpul PMAG® with black); Hawaii added to the short barrel rifle shipping restriction |