When discussing the Mk 18 / CQBR program, we can divide the history and discussion in to two distinct categories: 1) The weapon system itself / upper receiver supplied and 2) the group of accessories deployed on said weapon systems (SOPMOD).

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Mk 18 Mod 0 / CQBR

The M4 carbine and M16 are not ideally suited for all missions, so it was proposed that the modularity of the M16 series would allow a user to replace the upper receiver of an existing weapon with one more suitable to the task. One of two proposed special mission receivers that were planned for inclusion into the SOPMOD Block II kit, the CQBR has taken off on its own. Like the proposed Special Purpose Receiver, the Close Quarters Battle Receiver has been more or less taken on by the Naval Surface Warfare Center, Crane Division (often referred to as NSWC-Crane or just "Crane") as its own project following the CQBR's removal from the SOPMOD program. Just as the Special Purpose Receiver morphed into the Special Purpose Rifle, and was type-classified as Mk 12 Mod 0/1, the complete CQBR-equipped carbine has been type-classified as the Mk 18 Mod 0.

The purpose of the CQBR remains to provide operators with a weapon of submachine gun size, but firing a rifle cartridge, for scenarios such as VIP protection, urban warfare, and other close quarters battle (CQB) situations. The CQBR is designed to provide improvement over previous AR-15/M16-type weapons in this category. The CQBR is usually issued as a complete weapon system, and not just an upper receiver. The CQBR was once only available to Naval Special Warfare units, but the Mk 18 Mod 0 has become general issue for Visit, Board, Search, and Seizure (VBSS) missions and, as of 2006, for NCIS agents deploying to active combat zones.[citation needed] The Mk 18 is also used by the Coast Guard's Tactical Law Enforcement Teams, Maritime Safety and Security Teams, and Maritime Security Response Team and the United States Navy's Explosive Ordnance Disposal (EOD) Operators.

Wikipedia has a good article with sources on the original CQBR/Mk 18 Mod 0 and its development.

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SOPMOD

The SOPMOD Program Management Office at NSWC Crane, IN, provides standardized, versatile weapons accessories to meet needs across SOF mission scenarios. These accessories increase operator survivability and lethality by enhanced weapon performance, target acquisition, signature suppression, and fire control.

The CQBR itself was originally part of the SOPMOD program as an accessory replacement upper receiver for the M4. However it was eventually separated out into its own project.

SOPMOD accessories for the M4/CQBR originally included things like the KAC RIS/RAS, SOPMOD stock, KAC M4QD suppressor, AN/PEQ-2 illuminator/laser, ECOS-N red dot, ACOG, etc

The SOPMOD program has gone through multiple minor and major evolutions throughout its lifetime. The most significant is commonly defined as "Block II" and included the introduction/replacement of accessories that met the updated needs of operators. The most visually significant "Block II" accessory is probably the replacement of the KAC rail/FSB with the  Daniel Defense RIS II free float rail with low-profile gas block. Other updates include things like the Surefire SOCOM sound suppressor, EOTech RDS (553/SU-231 & EXPS 3-0/SU-231a), LA-5/PEQ ATPIAL, etc

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Link to docs and photo albums

 Presentation on CQBR from Crane
 Operator's Manual for Mk 18 Mod 0
 SOPMOD Program Overview presentation from Crane
 Photo Album of Mk 18 Mod 0
 Photo Album of CQBR with SOPMOD Block II accessories

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Because of the long history of both the CQBR/Mk 18 and SOPMOD programs, it's common to find weapons with various mixed configurations in the field, all depending on what time and unit the weapon system belongs to. Below are a few examples with descriptions to illustrate this.


Example of a Mk 18 Mod 0 in as-issued configuration including: M16A1 surplus lower receiver, SOPMOD buttstock, KAC RIS, KAC M4QD flash hider, Aimpoint Comp M2 in Wilcox mount, LMT rear fixed sight, CQD rear sling plate.


NSW Operator CQBRs with original SOPMOD accessories including KAC M4QD sound suppressor and AN/PEQ-2 IR Illuminator/Laser.


Member of a Combat Camera team with a CQBR with KAC RAS and M4QD flash hider but with other updated accessories from the "Block II" SOPMOD kit including SU-231a (EOTech EXPS 3-0), EOTech G33 Magnifier, LA-5/PEQ ATPIAL, Insight WMX-200.


Good example of "early" CQBR with SOPMOD Block II accessories including SU-231 (EOTech 553) and SU-233 (Insight M3x weaponlight).


Example of a more "modern" CQBR with SOPMOD Block II accessories including the updated SU-231a (EOTech EXPS 3-0) and Insight WMX-200 weaponlight.


Many times certain configurations are common within certain branches/groups. Here a Marine Raider is seen with a CQBR with an AN/PEQ-16 (USMC issued vs. the LA-5/PEQ SOCOM issued ATPIAL) and KAC NT4 suppressor (vs. the current Surefire SOCOM suppressor).

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Building a clone

If you're interested in building a "clone" of a Mk 18 or CQBR equipped M4A1, here is a basic parts list to use as a guide and get you started. This list is not exhaustive and see the * note above regarding personalized accessories.

                       | Mk 18 Mod 0                      | M4A1 CQBR Block I                   | M4A1 CQBR Block II                                      
Lower Receiver         | Mil-spec forged A1               | Mil-spec forged A2                  | Mil-spec forged A2                                      
Stock                  | Gen 1 SOPMOD (Black)             | M4, CAR, SOPMOD, CTR & others       | M4, CAR, SOPMOD, CTR & others                            
Pistol Grip            | A1                               | A2, ERGO & others                   | A2, ERGO & others                                        
Rear Sling Plate       | CQD                              | CQD                                 | CQD                                                      
Upper Receiver         | Mil-spec forged M4 profile       | Mil-spec forged M4 profile          | Mil-spec forged M4 profile                              
Barrel                 | 10.3"                            | 10.3"                               | 10.3"                                                    
Rail                   | KAC RIS                          | KAC RAS                             | DD Mk18 RIS II (FDE)                                    
Rear BUIS              | LMT Fixed                        | LMT Fixed, MATECH, & others         | MATECH, KAC 300m, & others                              
Front Sight            | A2 FSB                           | A2 FSB                              | KAC 99051 BUIS (Taupe)                                  
Front Sling Attachment | FSB Sling Swivel                 | FSB Sling Swivel                    | CQD                                                      
Muzzle Device          | KAC M4QD                         | KAC M4QD                            | SureFire FH-556-RC                                      
Supressor              | None                             | KAC QDSS NT4                        | SureFire SOCOM556-RC (FDE)                              
LAM                    | None                             | PEQ-2                               | LA-5                                                    
Weaponlight            | SureFire M962                    | SureFire M952, M962                 | Insight M3X (Tan), Insight WMX200 (Bronze)              
Optic                  | Aimpoint Comp M2 in Wilcox Mount | Aimpoint Comp M2 in QRP Mount, ACOG | EOTech 553 (Tan), EOTech EXPS3-0 (Tan), ELCAN 1-4x (FDE)

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Link to previous threads

 Previous Mk 18 / CQBR Thread Part I
 Previous Mk 18 / CQBR Thread Part II

Maybe CD? But he's probably still asleep 'cause it's tomorrow where he is.

Nice!

lol'd at that sneaky alphabet boi in the QR Code.

Any deals on EXPS3-0s right now?

EE?

I wouldn't know, but I've been told this is a correct, issued MK18 lower

I bet its fake

ADCO is still the beeezniiiiiis?

Sending my colt off to get choppy chopped next week

I suppose, as long as they are given proper specs.

Length measured from bolt face: 10.35"
Overall length: 10.75"
Thread length: .625"
Gas port: .070"

X2 for John Thomas at Retro Arms. Going to be sending him two uppers after next weekend. That guy is awesome

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CD

@combat_diver
In regards to the uppers with laser engraved numbers, who does that and what are they used for?

Thanks.

Crane did that for tracking the uppers. #1000xxxx were 10.3" uppers and #1400xxxx were 14.5" uppers.  No longer used in the Army anyway for tracking (property)

CD

He'll ya. Looks legit, and the ser. no. are on par with the ones CD posted. Looks period correct.

Not that you're very far off from this image, but which specs to use? I might go with retro arms works, I'll give them a call on Monday

The specs I listed are measurements I took from a factory Colt as well as a DD 10.3; Combat_Diver also measured a Crane cut Colt 10.3 and FN SOCOM 10.3 and confirmed the same specs.

Yeah, Retro Arms Works is probably the way to go. I've heard nothing but good things and he charges less from what I understand.

ETA: Technically, the DD 10.3" was 10.345" from the bolt face, but close enough ;p.

Roger that, I will go with the specs you provided then. Thank you

What should I put for OAL on my form 1? This is my first form 1, but I do have a quiet thingy in jail.

Using a B5 sopmod.

With 10.3 barrel, and gen1 LMT fully extended, I got ~29inches.

Submitted all of my 10.3 clones as 29.5 OAL.

Picked up the last Surefire SB at AU a few days ago. Now the long wait begins.

What muzzle device is everyone using if not the 4P? I still want to shoot it while I wait for the can but once I get it I doubt I will ever shoot it unsuppressed. I’m thinking flash hider now, muzzle brake later?

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Awesome!  It's my uncle's gun but he lets me shoot it every once in awhile.  I'm so glad it appears to be legit

I run an A2 Flash hider 'for now'. lol

Your uncle seems greedy, tell him to get some more legit looking gear for you to borrow.

I'd use whichever you prefer... for me, that'd be the flash hider. I wouldn't get too hung up on the baffles. I know that might sound strange coming from me since I kept a pretty close eye on the erosion of my RC early on. And I mean, yeah... no doubt that's a part of what determines can life. But it's only that, a part of it.

Despite some concern initially, I've since come to realize that maintaining can volume against carbon/lead/copper/bitmetal build up is of equal, if not far greater importance in the long term than the current state of your baffles. That in all likelihood, loss of volume is going to degrade performance long before baffle erosion. The suggestion that a sealed centerfire can is maintenance free and never needs to be cleaned is nonsense IMO.

Somewhere I read a post by Reed Knight talking about the NT4 that echoed exactly those sentiments. Basically, your can is going to clog up and lose volume before your baffles erode to the point the can loses significant performance. And I definitely believe it when it comes to the NT4, as the baffle design seems to lend itself to far faster carbon accumulation.

Reposts

NAVY 18-1 in Kuwait 16'

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Unissued marked uppers, Army SF Astan 18'
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CD

Thanks fellas!

Got the 552, magnifier and ecos-c back on the eod. Disregard the muzzle brake.

So that was you!?! I got some extra $ and I checked their website and was sorely disappointed.

ETA-At least it went to someone in here. I almost considered the mini monster!

Yeah that was me. I was updating my cart damn near everyday for the last few months to keep track of how many they had while I saved my funds. Got a little worried when their stock went from 12 to 3 within a few hours so I started eating a lot of pasta and ramen to get the last one. Lol. #clonelife

That’s good food for thought. I’ve always wondered how much a muzzle brake actually makes a difference vs. a flash hider.

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Looking good Wis

Thanks man! :)

Some not DoD "Mk18's". I'll gladly delete if this muddies the water too much.

DoS guns?

For suppression, a handful of tests on various mfg products show that the muzzle device doesn’t make a notable difference in suppression; often <1dB. But it does absolutely make an observable difference in terms of blast baffle erosion - probably even more so if your shooting with an aggressive firing schedule or  full auto. But like krdt is saying, does it really matter if using sacrificial muzzle brakes extends the life of your silencer from 15000 rounds to 20000 rounds versus a flash hider if if the can will clog up around 13000.***

***Those are completely made up numbers for illustrative purposes.

Where are you all getting your UID stickers?

Carolina laser works

A close up of the receiver would be nice if possible.
Also, are those D&D uppers on Colt lowers or all D&D rifles?

@ECPrevatte
Is the reason you have the tango qd latch on that side so you can undo it without taking you hand off the grip if ever needed? I have my latch on the same side as the ejection port because I see most here have it that way. But it being backwards like your makes more sense in the field.

That's one reason, but wasn't the original reason. For whatever reason, it was always annoying to me to be under the flashlight, so I would put it on the left.

Makes sense. Thanks