Black throughout — barrel and furniture. The gas block is not bolted on. It is part of the barrel.
- Barrel
- 16″ Monobloc®
- Handguard
- 15″ Light Weight M-LOK
- Weight
- About 6 lb
- Recoil
- VLTOR A5
- Price
- $1,750.00
// The Operator's Case
Most rifles are built to survive their weak points. This one does not have the usual one: the gas block cannot come loose, because it was never a separate part.
The SOTM4MB-LW pairs a 16″ Monobloc® barrel with the 15″ SOT Light Weight M-LOK Premium Line handguard. The rail covers nearly the whole barrel, so the support hand can go as far forward as the shooter wants and accessories mount where they are useful — but it does that without the mass a full-length rail normally brings.
The barrel underneath is the reason the line exists. On a Monobloc®, the gas block is milled into the barrel as one piece rather than clamped or pinned over the port. There is nothing to shift, nothing to rotate, and no alignment to lose — which removes the most common cause of a rifle that suddenly will not cycle. In 4150 CMV with a mid-length gas system.
The receiver set is ours on both halves — the SOTM4 billet upper on the SOTM4MOD2A5 billet lower, with fully ambidextrous controls fitted as standard, a VLTOR A5 recoil system, a PWS ratchet-lock castle nut, and B5 Systems stock, Type 23 P-Grip and storage plug on every configuration. The barrel is finished in Flat Dark Earth or Black Cerakote®, and the stock and grip come in Black, Black MultiCam or Flat Dark Earth.
On a conventional barrel the gas block is a separate part clamped, pinned or set-screwed over the gas port. It can shift, it can rotate, and if it does the rifle stops cycling — usually without warning and usually at the worst time. Every maker’s answer to that is a better clamp, a tighter pin, a stronger screw.
On a Monobloc® the gas block is machined as an integral part of the barrel. No clamp, no pin, no set screw, no alignment to lose — because the port and the block are one piece of steel.
That is what Bombproof by Design™ means, and this rifle is the clearest example of it we build. Not a stronger fastener holding a part in place, but a design where the part was never separate. A joint that does not exist cannot loosen, cannot rotate, and cannot be reassembled wrong — and it cannot be retrofitted to a conventional barrel at any price.
At about 6 lb this is a light 16-inch rifle by any measure. But a rifle does not feel heavy because of what it weighs — it feels heavy because of where that weight sits — and a handguard is the longest lever on the gun. Take mass off a 15-inch rail and the muzzle stops fighting you: the rifle settles faster after a transition, holds steadier offhand, and costs less to keep up over a long day. Trimming an ounce at the front is worth more than trimming three at the buffer.
Nearly the whole barrel is covered. That means a support hand that can go as far forward as the shooter wants, and every accessory mounted where it is actually useful rather than where the rail happens to end.
The SOTM4 billet upper on the SOTM4MOD2A5 billet lower — a matched set in the same profile, with the same styling and the same left-side relief cut. The receiver joint is not a place where two makers’ interpretations meet.
The rail is machined on every exterior surface from a custom 6061-T6 extrusion — not an extrusion with holes cut in it — which is where the flat, uniform surfaces come from. 11.3 oz complete with hardware, on a 15-inch rail.
4150 chrome-moly vanadium (CMV) with a mid-length gas system — a longer dwell, a gentler unlock, and a cycle that is easier on the brass and the bolt than a carbine-length port.
The VLTOR A5 extension runs a rifle-length spring and a longer buffer, giving the carrier further to travel before anything stops it. The impulse spreads over more time, which is what a shooter feels as a longer push instead of a shorter shove.
A Primary Weapons Systems ratchet-lock castle nut and end plate. It indexes into the end plate and stays there, so the back end does not rely on stake marks alone.
The Geissele G2S® is the only trigger offered on this rifle. Two-stage, non-adjustable, M4 curved bow — 2.75–3 lb first stage, 1.5–1.75 lb second, 4.25–4.75 lb total, and Geissele publishes those as approximate. The sears are wire-EDM cut from the same tool steel as the SSA®. There is nothing to set, which is the point. Fewer choices, made properly.
Ambidextrous selector, ambidextrous bolt release, and an extended paddle magazine release. Not an upgrade list — the SOTM4MOD2A5 lower ships with them installed and function checked.
The rifle shown carries the SOT proprietary billet serrated release button. Serrated because a bolt release is worked with a thumb, in gloves, without looking at it.
Where the light weight actually is
It is the handguard, and we would rather say so than imply the whole rifle was redesigned. The barrel is a full-profile 16″ Monobloc®, the receivers are the same ones we build everything on, and the A5 recoil system is heavier than a carbine setup, not lighter.
What changes is where the mass sits. Weight removed from a 15-inch lever at the front of the rifle does more for how it points and holds than the same weight removed anywhere else — and that is the whole argument for this configuration.
It comes in at about 6 lb. A 16-inch AR usually sits somewhere between six and a half and seven and a half, so the saving is real — but the reason it handles the way it does is where that weight is missing from, not the number on the scale.
If you want the lightest rifle on the scale, ask about a shorter barrel. If you want a 16-inch rifle that handles light, this is the one.
The rifle shown is this configuration
Black Cerakote® Monobloc® barrel, Black B5 Systems Bravo stock and Type 23 P-Grip. Photographed as it ships.
The other two configurations are the same rifle in a different finish — Flat Dark Earth, or Black MultiCam furniture on the same Black barrel.
// This Configuration
This is the Black configuration: a Black Cerakote® Monobloc® barrel with Black B5 Systems furniture. The plainest of the three, and the one that disappears fastest in a rack.
| SKU | MB-LW-B160-H150M-BLK-R |
|---|---|
| Barrel finish | Black Cerakote® |
| Furniture finish | Black — stock and grip as a matched pair |
The other two configurations are the same rifle in a different finish — same barrel, gas system, receiver set, recoil system, trigger and weight. Links at the foot of this sheet.
// Specification
| Product | SOTM4MB-LW Monobloc® Light Weight Rifle |
|---|---|
| SOT SKU | MB-LW-B160-H150M-BLK-R |
| Chambering | 5.56×45mm NATO — the Monobloc line is 5.56 only |
| Barrel | 16″ Monobloc®, 4150 chrome-moly vanadium (CMV) |
| Barrel finish | Flat Dark Earth or Black Cerakote® — by configuration |
| Gas block | Integral — milled as part of the barrel. No clamp, pin or set screw |
| Gas system | Mid-length |
| Handguard | 15″ SOT Light Weight M-LOK Premium Line, free float |
| Handguard material | 6061-T6 aluminum from a custom extrusion, hard-coat anodized. Every exterior surface machined, not just cut for slots |
| Handguard weight | 11.3 oz including mounting hardware |
| Barrel nut | 4140 steel, included |
| Handguard mounting | M-LOK® at 3, 6 and 9 o’clock; full-length top rail; built-in anti-rotation tabs |
| Upper receiver | SOTM4 billet upper — SOT profile with the left-side relief cut |
| Receiver set | SOTM4 billet upper matched to the SOTM4MOD2A5 billet lower |
| Lower receiver | SOTM4MOD2A5 |
| Magazine release | Extended paddle |
| Selector | Ambidextrous |
| Bolt release | Ambidextrous — SOT proprietary billet serrated release button shown |
| Receiver extension | VLTOR A5 |
| Castle nut and end plate | Primary Weapons Systems ratchet-lock |
| Stock | B5 Systems Bravo |
| Grip | B5 Systems Type 23 P-Grip — with storage plug |
| Furniture finish | Flat Dark Earth, Black or Black MultiCam — stock and grip supplied as a matched pair |
| Bolt carrier group | Enhanced HMB™ in black nitride standard; Microbest Super-Staked in hard chrome available as an upgrade |
| Trigger | Geissele G2S®, M4 curved bow — the only trigger offered on this rifle. Two-stage, non-adjustable, 4.25–4.75 lb total (approximate) |
| Muzzle device | Wilson Combat Q-Comp® — hybrid brake and compensator; suppressor mounts on request |
| Weight | Approximately 6 lb |
| Price | $1,750.00 |
Notice — configuration
- The Monobloc® line is 5.56×45mm NATO only. The wider SOTM4 platform supports other chamberings; this line does not.
- The Geissele G2S® is the only trigger offered on this configuration — it is not a shooter’s choice here, unlike the rest of the Monobloc® line. Bolt carrier group remains shooter’s choice.
- Trigger and bolt carrier group are function checked on the rifle before it ships.
- Two barrel finishes and three furniture colors. Flat Dark Earth or Black Cerakote® on the barrel; Black, Black MultiCam or Flat Dark Earth stock and grip. The finish is a choice, not a tier — the rifle underneath is identical.
- Billet receivers as a category — especially those with distinctive geometry — can occasionally show cosmetic gaps or fitment clashes at the receiver joint when paired with a receiver from a different manufacturer, because styling is not part of the mil-spec. On this rifle both halves are ours, so the question does not arise.
- Custom configurations are built to order — tell us what you want and we will quote it.
Warranty
Built, headspaced and function checked by Second of Ten and warranted as a complete rifle. Component manufacturers’ warranties pass through in full; where they run broader than ours, we act as the owner’s agent in a claim.
// Further Reading
SOTM4MB-LW — Flat Dark Earth Flat Dark Earth barrel with matching furniture SOTM4MB-LW — Black MultiCam Black Cerakote® barrel with B5 Systems Black MultiCam furniture Geissele G2S® — M4 Curved Bow The trigger on this rifle — two-stage, non-adjustable, $175.00 VLTOR A5 Receiver Extension The recoil system behind this rifle — seven positions, $62.95 Upper and Lower Parts Triggers, bolt carrier groups, buffer system parts and furnitureBombproof by Design™ and Gundaddy® are marks of Second of Ten LLC. Monobloc® is a mark of HM Defense; VLTOR®, A5™, Primary Weapons Systems, B5 Systems, Geissele Automatics, Wilson Combat and Cerakote® (a mark of NIC Industries, Inc.) are the property of their owners and are used here to identify components; their appearance indicates that Second of Ten incorporates the part, not sponsorship, affiliation or endorsement. Specifications are subject to change without notice. Firearms should be inspected and function checked by a qualified armorer before firing.
A joint that does not exist cannot fail.