12.5″ SOTM4MBA5 Forged Short Barrel Rifle, 11.7″ M-LOK
12.5″ Monobloc® · mid-length gas · forged MOD-0 upper and SOTM4 forged lower · A5 recoil system · B5 Systems Bravo
The Monobloc® short barrel rifle, on a forged receiver set.
CONFIGURATION SHOWN: 12.5″ SOTM4MBA5 Forged Short Barrel Rifle, 11.7″ M-LOK: 11.7″ M-LOK handguard, black, Magpul PMAG® engraved with the SOT crest, standard A2 Extended Flash Hider.
Mid-length gas at 12.5″, on a forged receiver set.
The SOTM4A5 forged short barrel rifle with a 12.5″ HM Defense Monobloc® barrel that reaches mid-length gas through precision port sizing, thermal fitted to the MOD-0 forged upper, on an SOTM4 forged lower, with an 11.7″ M-LOK handguard. Black, $1,450.00.
The same Monobloc® rifle, on a forged receiver set
The barrel, the gas system, the bolt and the platform are the Monobloc® line as SOT builds it on billet. What changes in the SOTM4A5 Forged Series is the receiver set: a MOD-0 forged upper and a forged SOTM4 lower in place of the billet pair. That is the whole difference, and the reason the price steps down.
What forged buys: strength
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 one, and tougher against impact and hard use. A forged MOD-0 pair is also held to Mil-Spec dimensions, so it fits like every other Mil-Spec receiver, and the Monobloc® barrel is thermal fitted to the upper, with no retaining compound and no bedding. What billet gives instead is tighter tolerances and design freedom.
The gas block is part of the barrel
On a Monobloc® 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 set-screw, and nothing to shift. The port is sized at manufacture for mid-length gas, even at 12.5 inches, which also means the gas cannot be tuned.
The HMB™ bolt, with SOT's marks
The HM Defense HMB™ bolt replaces the cam pin through-hole with a socket, keeping material at the bolt's most heavily loaded point. SOT adds an arrow index to the cam pin, so it always goes back in the same direction for better wear and cam pin life, and an index line across the staked gas key screws.
A two-stage trigger as standard
The Schmid Inside Elite two-stage in DLC is the standard trigger: 3.5 lb total, 2.0 lb to the wall and 1.5 lb to break. The Geissele G2S® or the LaRue Tactical MBT-2S can be installed instead, for $100.00 more. Second of Ten does not stone, polish or modify sear engagement surfaces.
Numbered stock positions
The SOT A5 buffer tube has nine positions, with numeric index marks 1 through 8 laser engraved on it. The B5 Systems Bravo stock has a window over the extension, so you can read the setting at a glance and return to it.
Two things this short barrel rifle does not have
It cannot take an adjustable gas block: gas is set by the port at manufacture. And there is no right-side bolt release: the bolt catch is the Geissele Maritime, a left-side control. The billet builds carry a right-side release; this one does not, by design.
| 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 |
Other muzzle devices and suppressor mounts on inquiry: we offer a wide variety, including mounts for many suppressor manufacturers and other Plan B options.
Where this short barrel rifle can ship
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. Not available in California, Connecticut, Delaware, Illinois, Maryland, Massachusetts, New Jersey, New York, Rhode Island, Washington or Washington, D.C. In Colorado and Vermont, and wherever magazine-capacity limits apply, it ships with compliant magazines instead of the 30-round magazine. Laws change: please confirm with your local 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 product page shows the current price and availability. Second of Ten LLC · Stuart, Florida · FFL 07 / SOT Class 02.
| Version | Date | Change |
|---|---|---|
| 1.2 | 2 NOV 2026 | Forged vs billet corrected: forged receivers are the stronger (grain follows the part); billet holds tighter tolerances. |
| 1.1 | 2 NOV 2026 | Lower controls corrected: Geissele Ambi Posi-Snap selector, button magazine release, nitride takedown and pivot pins (SOTM4MOD1 A5 lower); SOT A5 nine-position buffer tube. |
| 1.0 | 2 NOV 2026 | First issue |