12.5″ SOTM4MBA5 Forged Short Barrel Rifle, 11.7″ M-LOK

Description

The same Monobloc® short barrel rifle, on a forged receiver set. The barrel, the gas system and the bolt are the HM Defense Monobloc® line as Second of Ten builds it on billet. What changes is the receiver set: a MOD-0 forged upper and an SOTM4 forged lower. Forging makes the grain of the aluminum follow the shape of each receiver, so they are stronger and tougher against impact and hard use than billet, held to Mil-Spec dimensions, with the barrel thermal fitted to the upper: no retaining compound, no bedding. Bombproof by Design™.

Short barrel rifle. 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.

Open the zoom viewer, photographed as built: both sides.

Barrel

• 12.5″ HM Defense Monobloc®, standard bore, 4150 chrome-moly steel (CrMo), Black Cerakote®
• Match-grade, triple-honed, button-rifled bore
• 5.56×45mm NATO, .223 Remington compatible · 1:8 twist · M4 feed ramps · 1/2×28 muzzle thread
• Mid-length gas, sized at manufacture (not adjustable), reached at 12.5″ through precision port sizing, with the gas block milled straight into the metal of the barrel blank during manufacturing: no separate block to clamp, pin or shift
• Threaded barrel extension
• A2 Extended Flash Hider, Black Cerakote®, standard
• Optional add-on: 4-port hardened flash hider, Black Cerakote®, $40.00
• Optional add-on: Faxon MuzzLok® Gunner 3-port 5.56 steel muzzle brake, $60.00
• Optional add-on: Faxon MuzzLok® stainless steel Plan B muzzle brake, $75.00
• Other choices on inquiry: we offer a wide variety, including mounts for many suppressor manufacturers and other Plan B options. Contact us
• Suppressor-ready with any of the Faxon MuzzLok® brakes: the MuzzLok® removes to leave the 1/2×28 thread for a direct-mount suppressor

Upper Receiver

• SOT MOD-0 forged upper, 7075-T6, black anodized, barrel thermal fitted with no retaining compound and no bedding
• Forward assist
• SOT Enhanced HMB™ bolt carrier group, Black Nitride (standard): HM Defense HMB™ bolt with a cam pin socket in place of the through-hole
• Arrow-indexed cam pin on the standard group: always installed in the same direction for better wear and cam pin life
• Staked gas key with an SOT index line across its screws on the standard group
• Optional upgrade, in place of the standard group: Young Manufacturing M16 Chrome BCG with HMB™ bolt, $149.00, supplied as Young Manufacturing build it, without SOT’s enhancements: no arrow-indexed cam pin, no index-marked gas key, and the gas key is not staked
• Optional upgrade, in place of the standard group: Microbest Super-Stake C158 all-chrome BCG, heavy-duty stake, with chrome HMB™ bolt, $149.00
• Ejection port cover engraved and filled MONOBLOC · 5.56 NATO on both faces, exterior and interior
• Breek Arms WARHAMMER charging handle, ambidextrous latches

Handguard

• 11.7″ SOT Wedge-Lock M-LOK, free float, black anodized

Lower Receiver

• SOTM4 forged lower, black anodized, rollmarked MOD: SOT M4 · CAL: MULTI
• Schmid Inside Elite 2-Stage trigger, DLC, curved bow (SI-FCG-2S6-ELT), standard: 3.5 lb total, 2.0 lb first stage, 1.5 lb second
• Optional trigger, installed in place of the Schmid, $100.00: Geissele Automatics G2S® two-stage (4.25–4.75 lb total)
• Optional trigger, installed in place of the Schmid, $100.00: LaRue Tactical MBT-2S two-stage, curved or straight bow
• Second of Ten does not stone, polish or modify sear engagement surfaces
• Geissele Automatics Ambi Posi-Snap safety selector
• Geissele Automatics Maritime bolt catch (08-153), left side only: there is no right-side bolt release on this build
• SOTM4MOD1 A5 lower: button magazine release, nitride takedown and pivot pins
• B5 Systems Bravo aluminum trigger guard (ATG-1092), enlarged bow
• SOT A5 nine-position buffer tube, numeric index marks 1 through 8 laser engraved
• A5H2 buffer and A5 rifle-length action spring
• Forward Controls Design CN5F castle nut, staked, with Vibra-TITE VC-3 on the extension and nut threads
• B5 Systems quick-disconnect end plate (AEP-1090), single-point sling attachment

Furniture

• B5 Systems Bravo stock (BRV-1085), black, with an index window over the receiver extension
• B5 Systems Type 23 P-Grip, black

Extra Information

• Overall length 31.5″ with the stock collapsed, 34″ extended · weight unloaded 6 lb 6 oz
• Ships with one Magpul PMAG® 30 GEN M3™ 30-round magazine, laser engraved with the SOT crest
• Optional add-on: Magpul MBUS 3 front & rear flip-up sight set (MAG1166-MAG1167), black, $75.00
• Anodized shade varies from lot to lot
• Some lowers photographed carry SAFE / SEMI / AUTO selector markings for appearance only: every Second of Ten rifle is semi-automatic, with a semi-automatic trigger
• 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
• Laws change: please confirm with your local FFL before ordering
• Short barrel rifle · ships to your FFL dealer only

MONOBLOC® and HMB™ are marks of HM Defense, LLC, used under license. Geissele Automatics, LaRue Tactical, Schmid Inside, B5 Systems, Forward Controls Design, Breek Arms, Lancer Systems, Magpul and Faxon marks are the property of their owners. Their appearance indicates that Second of Ten incorporates the component, not sponsorship, affiliation or endorsement.

Open the sell sheet: the full specification, options and prices on one printable page.