M4A1 Block II Clone Build Guide: The RIS II Upgrade

Nobody was ever issued a rifle called the M4A1 Block II. Collectors use the name for a special operations M4A1 wearing the Daniel Defense RIS II rail and the accessories SOCOM bought in the second phase of its SOPMOD program.

What is the M4A1 Block II?

The Special Operations Peculiar Modification program, SOPMOD, never produced a rifle. Naval Surface Warfare Center Crane in Indiana ran it as a catalog of SOF carbine accessories bought in increments called blocks.

A 2007 Crane briefing dates Block II fielding to 2005 through 2010, but the parts took years longer to reach every team. You're copying an M4A1 from somewhere between 2006 and 2016.

The SOPMOD Block II program

Block II grew out of Crane's Miniature Day/Night Sight effort, with contracts from March 2004 to September 2005 and full rate production approved in May 2006.

The rail had its own competition. Crane gave A.R.M.S., Daniel Defense and Knight's Armament development contracts in March 2005. A 2009 notice named Daniel Defense the sole source, with rails in SOF hands since May 2006.

Daniel Defense says SOCOM wanted the rail to free float an M203 without extra parts or tools.

What Crane counted as Block II

Most clone lists treat the RIS II as the heart of Block II. Crane didn't.

On Crane's 2007 MDNS slide, the Block 2 box holds four items: the SU-230/PVS Elcan, the LA-5/PEQ laser, the SU-232/PAS thermal sight and the AN/PVS-24 night sight.

The rail sits with the Block 1 phased replacements, along with the EOTech, ACOG and light, though Daniel Defense markets it as built for SOPMOD II. Collectors call any M4A1 with the RIS II and this kit a Block II, and the same kit on the old Knight's rail a phased replacement.

Crane counts the Mk 12 as a SOPMOD client weapon too. See our Mk12 build from Lone Survivor.

Who carried the Block II M4A1

Crane's 2007 briefing lists Army Special Forces, the 75th Ranger Regiment, Navy SEALs, Air Force Special Tactics and Marine Corps special operations as users.

Room clearing at night in Nahr-e Saraj district, Helmand, Afghanistan, 14 August 2012. DVIDS tags the image 75th Ranger Regiment. The man at the door holds a Remington 870, with his long railed carbine slung. Photo: Spc. Justin Young, U.S. Army, via DVIDS 661579. Public domain.

Fielding was slow. Retired Special Forces soldier Jeff Gurwitch heard about the FSP rail in 2007, got one in 2014 and got the continuous rail in fall 2015.

The Reptile House blog dates full rollout across SOCOM to early 2016, Tier 1 units first. It says Army SF liked the FSP rail and was among the last to get the whole kit.

Live fire training with A Company, 2nd Battalion, 75th Ranger Regiment at Camp Roberts, California, 31 January 2014. The carbine in front wears a continuous quad rail, a folding front sight, an Insight light and an Elcan optic. The man behind has an Air Force TACP patch. Photo: Staff Sgt. Teddy Wade, U.S. Army, via Wikimedia Commons. Public domain.

We found no source on how often SEALs picked a 14.5 inch Block II over a Mk18, so we won't guess.

What changed from Block I

The rail, the laser, the day optic and, after 2011, the suppressor changed. The barrel and stock design carried over, and our M4A1 Block I clone build covers the carbine this one replaced.

The RIS II rail: FSP or continuous, FDE or black

Daniel Defense built the M4A1 RIS II in two versions, both 12.25 inches for carbine gas, in FDE or black. Wikipedia says 12.5, but every Daniel Defense page we checked says 12.25.

The 2009 notice asked for SOCOM approved Flat Dark Earth and gave no reason, and black rails served too. The M4A1 photographed at a National Museum of the Marine Corps facility in 2025 wears a black continuous rail with one FDE panel.

FSP first, continuous later: the two M4A1 rails

The FSP rail came first. A slot in its top rail clears the carbine's front sight base, so your barrel keeps its sight. Daniel Defense pairs the later continuous rail with the Mk12 low profile gas block.

A game wiki dates the two rails to 2007 and 2012, but the Crane brief it cites has neither date.

Want the earlier look? Start with the FSP card. Both cards are FDE rails, FSP first, and the BCM QRF-12 is a budget quad rail.

SOPMOD Block II parts list: the issued part and what to buy

A Block II clone has 25 slots. We checked each against one enthusiast parts list and our catalog, and graded the evidence for the issued part.

Product photography is supplied by the listed merchants through their affiliate feeds, laid out by GunPartPicker.

Exact means the issued part or its standard pattern is in our catalog, in stock or not. The evidence grade is ours.

The lower receiver group

Everything here but the stock is standard M4 pattern, so your clone work is mostly markings.

Lower receiver: Colt A2 profile with a UID label

The issued lower is a Colt M4A1 marked SAFE, SEMI and AUTO, and you can't own one. The enthusiast list asks for an A2 profile Colt with a UID label, and a soldier in Afghanistan in 2011 called his issued rifle a Colt.

Neither card is the issued part. The Colt LE6920 is an equivalent to use as a lower donor, and the PSA and Aero are budget lookalikes. The Aero is out of stock everywhere we track.

Stock: SOPMOD by LMT, then B5 Systems

The SOPMOD stock started at Crane, which patented it in 2002. LMT was the main maker for several years, and B5 Systems has been the main Defense Department maker since 2011, so a B5 isn't a lookalike.

Buy B5's Government Issue version, because the Enhanced SOPMOD adds steel reinforced QD sockets the issued pattern lacks. Both cards are the issued pattern, and the TacFire is a budget lookalike.

Trigger, grip and trigger guard: semi auto, A2, GI

No document we reached names a Block II trigger change. Treat the issued trigger as Colt's automatic group, and fit your semi auto lower with a mil spec semi auto trigger.

The museum rifle has an A2 grip and GI style guard, and those two cards are exact. The CMMG is the trigger equivalent. The enthusiast list names the Geissele SSA, a commercial semi auto trigger that no source ties to Block II, so we leave it off.

The upper receiver group

Pick the rail version first. It decides your gas block and front sight, and loosely the barrel profile.

Upper receiver: Colt flat top and collector forge marks

The enthusiast list wants a Colt upper with keyhole or square forge marks and a CAGE code. We couldn't confirm it, because the museum rifle's marks aren't legible and no document names them.

Colt uppers in our catalog come as complete groups, so your exact start is the 14.5 inch LE6921. The Expo Arms T marked upper is budget.

Barrel: 14.5 inch SOCOM or government profile

The issued barrel is 14.5 inches with a 1:7 twist and carbine gas. The heavy SOCOM profile has a cut so an M203 mount slips past, and it predates Block II. Gurwitch says 1999 and Wikipedia says 2004.

The profile under the continuous rail is disputed. A game wiki and one forum poster say government, and a retailer says SOCOM.

Every Colt SOCOM barrel in our feeds is out of stock. The LMT has a front sight base for the FSP rail, and the FN is a government profile. The BCM SOCOM upper is your in stock equivalent, and the Rosco is budget. Skip Daniel Defense's mid length barrel, which belongs to the URG-I.

Gas block: F marked front sight base or Mk12

FSP rifles use the carbine's F marked front sight base as the gas block. Continuous rail rifles use a Mk12 low profile block. Daniel Defense holds the Crane contract for it, and owners say Badger Ordnance builds to the same spec.

Owners also note that issued blocks are held by set screws, not pins. Check your barrel's gas journal against this .750 block. The last card is a carbine length gas tube, which either block needs.

Muzzle device: SureFire 4 prong, KAC NT4 or A2 birdcage

The 2011 SureFire award named the FH556-215A adapter, and SureFire's 2012 catalog shows it with open prongs. We don't stock it, so our equivalent is the SureFire SOCOM 4 prong FH556RC, named in a 2019 URG-I parts list.

Earlier rifles wore the flash hider that mounts Knight's NT4 can. Knight's sells that mount as the M4QD flash suppressor kit, P/N 93048. A2 birdcages show up too, on the AR15.COM chart and on a rifle issued in 2011.

The M4QD card is an exact variant. The BCM extended A2 is an alternative, not an issued part, and it runs longer than a standard A2, so measure before you pin anything.

BCG and charging handle: M16 carrier, GI or Gas Buster

The enthusiast list is the only source for the maker, naming Colt and FN. An M16 profile carrier is legal in a semi auto rifle, and Daniel Defense fits one to its own Block II style upper.

Gurwitch says the M4A1 kept the GI charging handle, but PRI's Gas Buster was in SOF use from 2004, so either is period correct. The BCM carrier is the in stock stand in for the Colt.

Sights and optics

Optics vary more than anything else on these rifles. The Elcan is the only day optic in Crane's Block 2 box.

Backup sights: front sight base, KAC or Matech

On the FSP rail, the front sight base is your front sight. Continuous rail rifles used a Knight's folding front sight, and the museum rifle's is black, marked P/N 99051.

AR15.COM posters in 2011 named KAC and Matech rear sights. Crane gave Knight's and A.R.M.S. Backup Iron Sight II contracts in 2005, and we couldn't find who won.

The black KAC front sight, P/N 99051, is the museum rifle's part, but it and the KAC 600 meter rear are out of stock. The Matech may not be the issued model, the KAC micro is an in stock equivalent and the Troy is budget.

Optic: Elcan SpecterDR 1x/4x, the SU-230

The SU-230/PVS is the Elcan SpecterDR 1x/4x, adopted in late 2007 per Gurwitch. Crane's 2007 briefing says SOPMOD topped it with a Docter mini red dot.

The Elcan card is a commercial SpecterDR, not an SU-230 labelled unit, so it's an equivalent. Our only Docter is a pistol version with no offers, and the Burris FastFire III is your budget top dot.

Optic: EOTech 553 or EXPS3-0, and the G33 magnifier

Crane's SU-231/PEQ was based on EOTech's 500 series, which clone lists call the 553. The SU-231A is the EXPS3-0. Gurwitch says both were still in wide SF use in 2017. The 553 is listed but was out of stock on 27 September, so the EXPS3-0 stands in for both.

No Crane document names a magnifier. The AR15.COM chart lists EOTech and Aimpoint 3x magnifiers, and the enthusiast list names the G33. Aimpoint's 3X-C is the in stock Aimpoint option, though no source names the model.

Other optics: ACOG, Aimpoint, Razor and Nightforce

The SU-237/PVS is a Trijicon ACOG with a Docter on top, and its NSN record names the TA31ECOS. Owners disagree on how often Block IIs carried one. Gurwitch says most SF teams got Aimpoint CompM4s around 2010 and then handed them back.

The enthusiast list adds Aimpoint T-1 and T-2, Vortex Razor, Leupold Mk6 and Nightforce optics with no unit or date, so we treat them as owner choices.

The CompM4 is exact and the T-2 comes from the enthusiast list. The FDE ACOG is the SU-237 equivalent, with an RMR where the Docter sat, and the TA31F is a budget ACOG.

Accessories: laser, light, suppressor and grip

Most issued accessories are military only or missing from retail, so your clone leans on stand ins here.

Laser and light: PEQ-15, LA-5 and Insight M3X

The Block II laser was the LA-5/PEQ, SOCOM's high power AN/PEQ-15, and it replaced separate PEQ-2 and PEQ-5 units. Our catalog lists the LA-5 as law enforcement only, so the Steiner DBAL-A3 stands in and the Holosun is budget.

The light was the Insight M3X, or SU-233/PVS, until the LED WMX200 replaced it in late 2010. Neither Insight is in our feeds, so the first card is a SureFire Scout, the light family the enthusiast list names without a model.

Suppressor: Knight's NT4 or SureFire SOCOM

In 2011 Crane gave SureFire the Family of Muzzle Brake Suppressors award for the FA556-212-DE can and FH556-215A adapter. Gurwitch says the SureFire replaced a Knight's can with more blowback and weight and a lower round count.

The Knight's can didn't vanish, though: the museum rifle carries one consistent with the NT4.

Knight's NT4 is listed in black and flat dark earth, both out of stock on 27 September. We don't carry the SureFire contract can, so both SureFire cards are the SOCOM556-RC2, and no source ties the RC line to the contract can. The newer NT4 PRG is in stock, but it isn't an issued can, and we couldn't confirm it fits the M4QD hider. Federally it needs no approval since October 9, 2026, but check your state.

Foregrip and sling mount: TangoDown and CQD

The AR15.COM chart lists a tan TangoDown grip, and the museum rifle's FDE grip has tangodownllc.com moulded in. The two TangoDown cards are exact. One SF soldier ran a Magpul AFG from about 2008, and the Daniel Defense grip is budget.

The enthusiast list names the CQD forward sling mount, but unit photos in The Reptile House show Viking Tactics and Daniel Defense mounts too. We don't stock the CQD yet, so the Viking Tactics and Daniel Defense rail mounts are equivalents, though we couldn't match the exact models in the photos.

Clip on night sights, M203 and magazines

The AN/PVS-24 image intensified clip on and SU-232/PAS thermal clip on were Block 2 items, and the RIS II carried the M203. No merchant we track sells any of them.

No document names a Block II magazine. Crane's topmaps list High Reliability Magazines and the museum rifle has a PMAG. Colt's USGI aluminum magazine is the standard pattern, Brownells' aluminum mag is the equivalent and the PMAG matches the museum rifle.

M4A1 Block II variants over its service life

A Block II carbine kept changing while it served, so your clone is a snapshot of one moment. Early rifles carried the new accessories on the Knight's RAS, then came the FSP rail, the continuous rail and, after 2011, SureFire cans.

A 1st Battalion, 10th Special Forces Group soldier on a range in Germany, 23 January 2015. His M4A1 still has the fixed front sight base and a short rail ending at it, with an Elcan and a tan light added: a transitional setup, not the RIS II. Photo: Visual Information Specialist Adam Sanders, U.S. Army, via Wikimedia Commons. Public domain.

CQBR Block II and URG-I: the same kit, other uppers

Our CQBR Block II clone build covers the short barreled sibling, and the Mk18 Mod 0 clone build and CQBR Block I clone build cover what came before it.

From 2018 USASOC swapped the RIS II and SOCOM barrel for a Geissele Mk16 rail and a Daniel Defense mid length barrel. The accessories stayed, so collectors still say Block II. Our M4A1 URGI clone build covers that upper.

For a rifle from the same years outside SOCOM, see our 13 Hours contractor carbine.

What you can buy today and what is unobtainable

We matched all 25 parts on our Block II list against the catalog on 27 September 2026. Fifteen are the exact part or its standard pattern, though several are out of stock everywhere we track.

Equivalent covers stand ins for parts you can't buy too: the laser, light, suppressor and a 37 mm flare launcher that isn't an M203 lookalike.

Complete Block II uppers and rifles on the market

A complete upper is your fastest route. Daniel Defense's M4A1 FSP upper page shows an F marked front sight base and an M16 profile carrier.

The last card is a complete Type A rifle, listed as a 14.5 inch barrel pinned and welded to 16. We haven't checked either against the issued list, so treat them as near clones and read each listing before you buy.

Military only and missing parts

You can't buy the select fire lower or its automatic trigger parts, and our merchants list the LA-5 lasers for law enforcement only. No merchant we track sells the clip on night sights or an M203.

We don't have product pages yet for the FH556-215A adapter or the CQD sling mount, and Knight's NT4 is listed but out of stock. The Insight M3X, the WMX200 and the FA556-212-DE aren't in any of our feeds.

What an M4A1 Block II clone costs today

We priced each pick at its lowest in stock offer in our merchant feeds on 27 September 2026, and the cards show you today's prices.

A Colt donor lower with a B5 stock, the complete FSP upper, an EXPS3-0, the Scout light, the DBAL-A3, a TangoDown grip and a Matech sight come to $5,319.30. With the Elcan kit instead, you're at $7,974.31.

Neither covers the suppressor, shipping or pin and weld work, and the donor leaves you a spare 16 inch Colt upper.

M4A1 Block II builds on GPP: what they show

You'll find Block II parts in plenty of GPP builds that aren't clones.

From 31 January to 27 September 2026, builders added B5 SOPMOD stocks 527 times across 303 sessions and Daniel Defense M4A1 uppers 387 times at a median $1,499. Those adds come from every builder, and our add data can't say which came from this guide.

Over the same dates, the Elcan drew 5 adds from 3 sessions, so few of you pay for the SU-230 look.

Rail and gas system in GPP's RIS II builds

We found 21 builds with an M4A1 RIS II from 21 owners, created 22 December 2025 to 15 September 2026, and only 6 of them pass our stricter quality filter. We left out carts our seeding script made.

Across those 21 builds from December 2025 to September 2026, the black continuous rail leads the handguard slot in 7. In the same builds, a Daniel Defense mid length government barrel leads the barrel slot at 3 of 18, and a WMD mid length gas tube sits in 4 of 11 builds with one.

One builder's cart, named Block2, pairs mid length gas with an ACOG and a law enforcement only ATPIAL listing: a modern take, not a period clone.

The civilian legal path to an M4A1 Block II clone

Pinned and welded to 16 inches

A rifle barrel under 16 inches makes a short barreled rifle under the NFA. The usual clone fix is a muzzle device pinned and welded on to reach 16 inches.

AR15.COM owners warn a welded device makes the FSP rail hard to remove, while the continuous rail still slides off. Our AR-15 barrel length guide covers the wider tradeoffs.

Or run the bare 14.5 inch barrel as an SBR

The other route keeps the bare 14.5 inch barrel as a short barreled rifle. Since October 9, 2026, ATF no longer requires approval or registration for that, but some states ban SBRs or only allow registered ones, so check yours first.

If you file an optional Form 1 anyway, our NFA paperwork guide covers the ATF's proposed changes to photos and fingerprint cards.

Semi auto parts, suppressors and IR lasers

Keep the lower semi auto. You can't add an auto sear, automatic hammer, trigger or disconnector to a civilian lower, though an M16 profile carrier is fine.

Since October 9, 2026, neither the suppressor nor the SBR needs federal approval, though your state may still want them registered. Our suppressor tax and paperwork guide covers the tax, and the LA-5 listings in our catalog are law enforcement only.

What the internet gets wrong about the M4A1 Block II

You'll see Block II lore copied from site to site, and until this rewrite our guide copied some of it.

No source puts the Geissele SSA in the Block II kit

Our earlier version said the SSA was formally part of the Block II kit. Neither Crane briefing lists a trigger in Block II.

The SSA is a commercial semi auto trigger. Geissele's select fire SSF was developed for US SOF, but nothing ties it to Block II either.

The Block II light was an Insight, not a SureFire

Our earlier version named the SureFire M600, and Charlie's Custom Clones names the M900 or M952. Crane's 2006 briefing says the SU-233/PVS is based on the Insight M3X, and Gurwitch says the WMX200 replaced it in late 2010.

What our earlier version of this guide got wrong

The version that went live on 16 September 2026 had more errors than those two. We rewrote it on 27 September 2026, and here are the rest.

Sources and photo credits

Contract notices, Crane briefings and maker pages carry the dates and part numbers. Forum, blog and enthusiast sources are labelled.