CQBR Block I Clone Build Guide: Every Part You Need

In May 2003, Jeff Johnson of Naval Surface Warfare Center Crane briefed a defense industry symposium on a shortened M4A1. Its barrel was cut to 10.3 inches, its gas port opened to 0.070 inch, and a Knight's Armament M4QD flash hider took a suppressor.

Clone builders call that upper the CQBR Block I, and you can still build a close copy.

What a CQBR Block I clone copies

The Close Quarters Battle Receiver is an upper built to drop onto M4A1 lowers units already had. Your clone copies the early and middle 2000s version: Colt flat top, 10.3 inch chrome lined barrel, fixed front sight base, KAC quad rail and M4QD.

Small Arms Review wrote in 2008 that there's no standard Mk 18 configuration, so pick the year you want to copy.

The origin of the CQBR

Crane's 2003 briefing gave four aims: a shorter, lighter primary weapon, the standard carbine's reliability, working SOPMOD accessories and parts already in the supply system. GlobalSecurity says the CQB component split from the SOPMOD kit in 2003 and became the Mk 18.

Cut from 14.5 inch M4 barrels: 10.5, then 10.3

Crane started by cutting down Colt 14.5 inch M4 barrels. Small Arms Review says the first cut was 10.5 inches, then 10.3, the shortest length that could still take the KAC suppressor, and Colt later made barrels to Crane's spec. SAR is our only source for the 10.5 inch step.

Crane's 2003 test: 41,340 rounds through four uppers

Crane billed it as 40,000 rounds through four uppers and fired 41,340 rounds of M855, 10 percent suppressed, for a cumulative 738 mean rounds between stoppages. Gas tubes broke on three of the four, at 6,400, 7,200 and 9,100 rounds. Pack a spare gas tube.

Colt takes over: the 2004 sole source solicitation

In May 2004 Crane announced an intended sole source contract with Colt for up to 2,000 M4A1 carbines with CQBR uppers and up to 5,000 CQBRs. A solicitation states intent, and we found no award record. Small Arms Review says Colt was building the complete uppers by 2008.

CQBR Block I timeline, 2000 to 2016

Block 1 CQBR: what the name means

Only some of this upper's names came from the Navy.

Block I is a clone name borrowed from SOPMOD

No Crane or contract document we read says CQBR Block I. Crane wrote CQB Receiver or CQBR. Block I is borrowed from the SOPMOD accessory blocks, which Crane dated 1995 to 2005 for Block 1 and 2005 to 2010 for Block II.

Mk 18 Mod 0 names the complete carbine, built by Crane on reworked M16A1 lowers and by Colt on new M4A1 lowers. Our Mk18 Mod 0 clone build covers that rifle, and the CQBR Block II with the Daniel Defense RIS II covers what replaced this upper.

How to spot a Block I: FSB, KAC rail, M4QD

Look for a fixed front sight base, a Knight's Armament quad rail running back to the delta ring and a KAC M4QD flash hider. Don't trust rail slot numbers to tell you a RIS from a RAS, because two posters in one 2006 thread gave opposite rules.

A Navy SEAL in desert terrain, 11 January 2012, with a carbine in the Block I layout: fixed front sight base, KAC quad rail to the delta ring, KAC style flash hider, red dot, and a Colt rollmark with UID labels. No official caption names the weapon, and this is a late service photo. U.S. Navy photo (VIRIN 120111-N-KK330-549), public domain, via Wikimedia Commons.

Who carried the CQBR Block I

Navy boarding teams have the best paper trail.

SEAL teams and Army special operations

Naval Special Warfare and Army special operations units, Special Forces among them, put CQBR uppers on M4A1s they already had. SEALs more often ran it as a second upper than as a complete Mk 18.

If you want SEAL rifles on screen, see the Act of Valor SEAL carbine and the Lone Survivor Mk12 build from the Red Wings era.

Navy VBSS boarding teams and EOD units

Navy photos show USS Carter Hall's boarding teams issued MK-18 rifles in early 2006 and a USS Bulkeley team training with them in 2008. Small Arms Review adds EOD and boat security teams.

In August 2008 the Navy said Crane had sent more than 1,300 CQBR equipped M4A1s to VBSS sailors that summer. That conflicts with one veteran's memory of fleet rifles moving to 14.5 inch barrels, so we treat his account as disputed.

MARSOC, Force Recon and Rangers: reported, not documented

Our earlier version said all three fielded this upper, and we found no primary source for any of them. A 2014 forum post shows a Marine Raider CQBR with a PEQ-16. Coast Guard and NCIS use appears only in secondary lists.

The complete parts list

Evidence runs strong when Crane, Navy, maker or trade press records back a part, and weak when only forum posts do. We cross checked every slot against one enthusiast parts list.

Product photography is supplied by the listed merchants through their affiliate feeds, laid out by GunPartPicker. The Colt upper, KAC RAS and KAC 300 meter sight on this board were out of stock on 27 September 2026.

The upper receiver group

The CQBR is the upper itself, so start here.

Upper receiver: Colt M4 flat top and its forge marks

The issued receiver is a Colt M4 flat top, and clone lists cite AF, keyhole and square forge marks. One VBSS veteran says early Mod 0 uppers were cut down M4 uppers, often Diemaco or Anchor Harvey forgings.

Colt's M4 upper is listed but out of stock. LMT's is the in stock equivalent, and the Wilson forging is the budget pick without Colt marks.

Barrel: Colt 10.3 inch with the 0.070 inch gas port

The issued Colt barrel is 10.3 inches, chrome lined, 1 in 7, carbine gas, with the 0.070 inch port. Small Arms Review calls it light contour, and clone lists say government profile.

Colt's M4CQB 10.3 inch barrel assembly is the closest you can buy, though we couldn't confirm its port size. The LMT card is the 10.5 inch length SAR mentions, and the Rosco is a budget 10.3.

Gas block and front sight: the Colt F marked FSB

Block I uppers keep carbine gas and the F marked front sight base. The KAC RAS needs both, plus a delta ring and a round handguard cap. If you buy a bare barrel, these cards cover the gas tube, post and F marked base, though none is Colt made.

Muzzle device: KAC M4QD, the mount for the NT4

Crane's slide reads "Standard flash hider replaced with M4QD flash hider for suppressor compatibility." KAC still sells the hider as kit 93048, threaded 1/2x28, and lists the QDSS-NT4 as its can.

A forum poster says the 2005 and 2008 Mk 18 repair standards show an A2 compensator, hence the birdcage card. We found no period photo of the B.E. Meyers 249F that the enthusiast list names, and we don't stock it.

Handguard: KAC M4 RAS, or the RIS on early uppers

The earliest uppers carried the KAC M4 RIS, and the M4 RAS (P/N 98064) served alongside it. Small Arms Review says Colt's Navy configuration used the RAS. Neither free floats: the RIS rides between the handguard cap and delta ring, and the RAS adds a barrel nut clamp.

Both RAS offers ($280.73 and $284.99) are out of stock today, and the RIS isn't in our feeds. The UTG rail is a budget lookalike.

Bolt carrier group: M16 carrier with reliability parts

The issued carrier is the full auto M16 pattern, legal in a semi auto AR under federal law. Crane added a five coil extractor spring and one piece gas rings, and Small Arms Review says Colt CQBR bolts carry a black extractor buffer, heavy gold spring and O-ring.

Colt's M16 group is out of stock, so the BCM is your equivalent. Expo Arms is the budget carrier.

Charging handle: GI issue, PRI Gas Buster optional

The forum poster with the 2005 and 2008 repair standards says they show a GI handle. He also saw PRI M84 Gas Busters with the military latch on SOCOM rifles in 2014, so save one for a late build.

The lower receiver group

Crane changed nothing on the lower, so yours only has to look right and stay legal.

Lower receiver: Colt A2 profile M4A1, semi auto for clones

The issued host was a Colt M4A1 select fire lower, a machine gun receiver no civilian can register new. The enthusiast list calls for a Colt A2 profile lower with a UID label, but DoD labeled older equipment too, so a label dates the marking, not the rifle.

Colt's semi auto LE lower has no offers, so H&R's A2 profile lower is your equivalent and PSA is the budget pick.

Trigger: issued Colt select fire, or a Geissele SSA

You need a semi auto group, and the CMMG and ALG kits give you the plain GI look. The enthusiast list names the Geissele SSA and SSA-E, but Geissele's own names make them semi auto, so they're a clone choice, never an issued part.

Stock: LMT SOPMOD Crane stock or the Colt M4 stock

A December 2005 Crane notice sought LMT's cheekweld buttstock, P/N L7LA2B, the SOPMOD stock people call the Crane stock. Nothing primary ties it to the CQBR, and Crane's 2003 photo shows a plain M4 stock, so either fits.

LMT sells the L7LA2BA today. Colt's M4 stock has no offers, and the B5 and Brownells stocks are budget lookalikes. A retailer's claim that B5 later won the SOPMOD stock contract is unverified.

Grip and receiver extension: A2 and six position

Crane's photo shows an A2 grip, so any A2 grip is correct, and clone lists add Ergo grips as a user choice. Small Arms Review says six position receiver extension, Wikipedia says four, and we go with SAR.

Trigger guard and CQD rear sling mount

The enthusiast list calls for a GI trigger guard, and PSA's USGI guard is that pattern. A VBSS veteran says every Mod 0 had a Close Quarters Defense rear sling mount. Small Arms Review confirms a one point end plate on the reworked lowers but names no maker. We don't stock the CQD mount, so this end plate stands in.

CQBR Block I sights and optics

The 14.5 inch carbine carried the same accessories, so see the SOPMOD Block I kit on the 14.5 inch M4A1 for the shared parts.

Rear sight: LMT L8A fixed, the one Crane ordered

In April 2005 Crane sought 208 plus 480 LMT L8A sights under the title "Rear Sight for CQBR Upper Receiver," after 500 in 2004. Earlier rifles wore a carry handle or KAC sights. Wikipedia gives the KAC 300 meter sight as the Block I kit sight, and Small Arms Review mentions Matech sights.

We don't stock the L8A, so the Daniel Defense fixed sight is your closest stand in.

Optic: Aimpoint CompM2, the SOPMOD ECOS-N

The SOPMOD red dot was the ECOS-N, a CompM2 variant, and Crane's 2003 overview lists it as a Block I phased replacement. The Army calls the CompM2 the M68. Clone lists add a Wilcox mount, which nothing primary confirms, and the only one in our feeds comes in a reproduction kit.

None of our merchants sells a new CompM2. The CompM4, sold as the Army's M68 CCO, is the closest current Aimpoint, the American Defense mount fits a used CompM2, and the PRO is your budget route. Our feeds also list an out of stock ECOS-N reproduction kit, an Aimpoint PRO on a Wilcox mount, and it isn't an issued sight.

Optic: Trijicon ACOG TA01 and RX01 reflex

Small Arms Review says the SOPMOD ACOG and the M68 were the most common Mk 18 optics, yet the enthusiast list skips the ACOG. The TA01 ACOG and RX01 reflex were SOPMOD Block I items, and we stock TA01s but not the RX01.

Later optics: EOTech 553, EXPS3-0, SpecterDR and G33

The EOTech 553 (SU-231/PEQ) arrived mid life as a Block 1 phased replacement. The EXPS3-0, ELCAN SpecterDR and G33 magnifier came later, so put them on a late rifle only. The 553 is listed but out of stock, so the 552 stands in.

Light, laser, suppressor and foregrip

You can't buy the original light or laser, and the can is an NFA item, so check your state.

Light: Insight VLI, then the SureFire type VBL II

The SOPMOD Block I light was the Insight Visible Light Illuminator, then the Visible Bright Light II, whose NSN carries P/N M952NXM07, matching SureFire's M952. No kit list we read names the M951 from clone lists. None is in our feeds, so the SureFire Scout stands in.

Laser: AN/PEQ-2 and PEQ-5, plus civilian stand ins

Block I rifles carried the AN/PEQ-2 or PEQ-2A and the AN/PEQ-5 visible laser, and veterans say many fleet rifles carried none. Full power IR lasers are restricted, so these cards are civilian stand ins. Our guide to civilian legal IR lasers explains your options.

Suppressor: KAC M4QD can and the QDSS-NT4

The can was KAC's quick detach suppressor, first the black M4QD and then the tan QDSS-NT4 (P/N 96062-2), both on the M4QD hider. KAC's current 556QDC won't fit that hider. The QDSS-NT4 with its M4QD hider is listed in black and flat dark earth, both out of stock, and fleet rifles usually ran without a can. The newer NT4 PRG is in stock, but it isn't the issued can, and we couldn't confirm it fits the M4QD hider.

For a modern short suppressed carbine instead, see the Sicario task force build.

Foregrip: KAC vertical grip, GripPod or TangoDown

The SOPMOD Block I grip was KAC's vertical foregrip, and Navy EOD bought 1,100 tan GripPods, per Small Arms Review. KAC's current Picatinny vertical grip is listed but out of stock, and the military model GripPod is in stock in tan and black. TangoDown is a variant on the enthusiast list, and the Daniel Defense grip is a budget stand in.

Front sling mount, sling and magazines

The issued front mount was the FSB sling swivel. Wikipedia lists the TOE Improved Combat Sling in the Block I kit, which we don't stock, so the Magpul MS3 stands in. Magazines were 30 round STANAG, so Colt's USGI aluminum magazine is the closest match. Brownells' aluminum mags are equivalents, and a PMAG is the budget pick.

CQBR Block I variants over its service life

There was no single correct Block I. The Block II replaced it, the later M4A1 URGI upper followed, and the 14.5 inch counterpart of the Block II is our M4A1 Block II clone build.

Which parts fit early, late and EOD Block I uppers

Early uppers: Crane cut barrels and the KAC RIS

Early uppers paired Crane cut Colt barrels with the KAC RIS. Forum posters in 2006 read the rail in the January 2006 Carter Hall photo as a RIS on an older engraved lower, which makes that rifle a Mk 18 Mod 0.

Colt built uppers: RAS rail and LMT rear sight

The enthusiast list copies this version. Small Arms Review puts the KAC RAS on it, and Crane's 2005 notice ties the LMT L8A rear sight to the CQBR by name. Add an M4QD, the LMT stock and an ECOS-N or ACOG.

Late KAC rail rifles in the 2012 Navy photos

The January 2012 SEAL training photos on this page show KAC railed carbines with fixed front sights, red dots, lights and vertical grips. Use them if you're building a late Block I.

Navy SEALs training on land and in water in California, 9 January 2012. The carbine carries a fixed front sight base, a weapon light, a vertical foregrip, a red dot and a suppressor. U.S. Navy photo by Mass Communication Specialist 2nd Class Martin L. Carey (VIRIN 120109-N-OT964-1266), public domain, via Wikimedia Commons.

Navy EOD rifles: VLTOR rail, M68 and Magpul stock

Small Arms Review documents an EOD rifle with a tan VLTOR CASV-EL rail, M68s on LaRue mounts, A.R.M.S. #40 sights, Magpul M93B stocks and tan GripPods. We don't stock the VLTOR rail, the M93B or a LaRue Aimpoint mount, so you'd be hunting those used.

What you can buy today and what is unobtainable

On 27 September 2026, 11 of 25 primary slots had the exact part in stock in our feeds, though six of those are generic mil-spec pieces like the A2 grip.

CQBR Block I parts in stock on GPP today

You can order the Colt M4CQB barrel, the KAC M4QD kit and the LMT SOPMOD stock right now. The Matech sight, TA01 ACOGs, Colt USGI magazines and the military model GripPod are in stock too, and so are the later EXPS3-0, SpecterDR and G33.

Out of stock or secondhand: KAC RAS, RIS and CompM2

The KAC M4 RAS, Colt M4 upper, Colt M16 carrier, QDSS-NT4 and EOTech 553 are listed but out of stock. The RIS and a new CompM2 aren't in any feed, so you're shopping used.

Parts no retailer can sell you

Select fire parts are machine gun parts. Full power PEQ lasers are military items, the Insight VLI and VBL II appear in no feed, and the original black M4QD can is a secondhand NFA transfer at best.

Build cost estimate

We priced each slot from the lowest in stock offer on 27 September 2026. Closest match takes the exact part when it's in stock and the nearest stand in when it isn't, and budget takes the cheapest listed part.

That's about $2,825 for the closest match and $1,545 for the budget build. Neither includes the KAC RAS, a lower parts kit, buffer and spring, the suppressor or a laser.

CQBR builds on GPP: what they show

Our data here is thin, and we'll say so. In the 90 days to 27 September 2026, site searches containing mk18 numbered 114 from 42 sessions. Nobody searched for CQBR, M4QD or NT4.

Seven Mk18 builds, zero Block I signature parts

Seven GPP builds, created January to June 2026, carry Mk18 in the name. None holds a KAC RAS, M4QD, Colt 10.3 inch barrel or A2 front sight base. Of the seven saved January to June 2026, three have a 10.3 inch part and two have Daniel Defense RIS II parts. Seven builds is a small sample.

No GPP build combines the RAS, Colt barrel and M4QD

Across every GPP build saved through 27 September 2026, the KAC RAS appears in 8 and the M4QD kit in 4. In that same span, Colt's M4CQB barrel shows up in 4 builds as an assembly and 5 stripped. No build pairs the RAS with either, so finish yours and you'll own the first.

The civilian legal path to a CQBR Block I clone

The SBR rules since October 9, 2026

With a stock, a 10.3 inch barrel makes a short barreled rifle. Since October 9, 2026, ATF no longer enforces the Form 1 or Form 4 approval and registration for SBRs, so federally you can assemble it today. Filing is optional and costs $0, and it's worth doing if your state only allows registered SBRs, or as cover in case the policy is reversed.

Taking an SBR across state lines, registered or not, still needs prior ATF approval on Form 5320.20.

Pistol builds and the changing brace rule

Without a stock, the upper can go on a pistol lower, but ATF's brace position has changed through rulemaking and litigation. Check it before you rely on a brace; our AR-15 pistol brace guide tracks it.

Why a 16 inch barrel stops being a CQBR

A 16 inch barrel skips the SBR question, but then you haven't built a CQBR. Pinning and welding a device to reach 16 inches isn't practical from 10.3 inches either.

Semi auto fire control parts only

Use a semi auto group. Never install an auto sear, M16 hammer, trigger, disconnector or selector. A full auto profile M16 carrier is fine under federal law.

Suppressors, IR lasers and state law

An NT4 or M4QD can no longer needs federal approval since October 9, 2026, though a Form 4 is still optional. Our guide to what changed for suppressors in 2026 covers the tax side. Civilians use Class 1 IR lasers or non emitting replicas, and your state can be stricter on all of it.

What the internet gets wrong about the CQBR Block I

Several of these errors sat in our own earlier version.

10.3 inches was set by the suppressor, not the gas system

Small Arms Review says 10.3 inches was the shortest length that could take the KAC suppressor. Reliability came from the 0.070 inch port, extractor spring and gas rings.

Neither KAC rail is free float

Both the RIS and the RAS are drop in rails retained by the delta ring. Our old FAQ wrongly gave the RIS its own barrel nut.

The M4QD is not the three prong QDC hider

The three prong device is KAC's later 5.56 QDC flash eliminator, which locks cans with a bearing collar. QDSS cans latch into the M4QD's groove, so by design the two don't interchange.

Geissele SSA: a clone choice, never the issued trigger

The SSA and SSA-E are semi auto, so they couldn't serve a select fire M4A1. The one Geissele report on these rifles describes select fire SSFs around 2014.

SU-232 is a thermal sight, not the CompM2

Crane's 2007 list calls the SU-232/PAS a thermal sight. The SOPMOD CompM2 was the ECOS-N, and the SU-231/PEQ is the EOTech 553.

No SOPMOD kit list names the SureFire M951

The M951 turns up in clone lists only. The Block I kit light was the Insight VLI, then the SureFire type VBL II.

The Block I did not retire in 2012

No source gives an end date. The Navy fielded CQBR carbines to VBSS crews in 2008, and one forum poster says he turned 17 Mod 0s in to Crane in 2016.

What our earlier version of this guide got wrong

We first published this guide with no sources. Here's what we said and what the record shows.

Sources and photo credits

We read every source below for this rewrite.

Photo credits