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

Is my Warcomp 3p acceptable or should I just get the normal 3p until funds allow for the 4 prong?
Edit added pic forgot about page ownage

So who is rocking a fully painted CQBR but has the rail still FDE? I think I’m going to turn my 300BLK into a CQBR.

Also anyone know what color cerakote is used on the RC?

3P or Closed Tine FH until 4P

The 3 prong is pretty acceptable as a substitute for the 4 prong.  So Id say switch if you weren't planning on the 4 prong, but since you are saving, put that money towards getting the 4 prong quicker.  Of course unless it will drive you crazy, which those things sometimes do for me.

Thanks for the reply guys. I will just return/exchange it for the 3p

I need to find a weekend to drink a few and spray my lower. I just know it would never come out as nice as some on here.
Is step one always to get enough liquid courage?

Less is more. Just spray lightly and let it wear.

That’s the plan. I just can’t decide if I want to do the lower and stock(Black sopmod) or upper and lower along with the stock. Definitely know that I do not want to do the fde rail.

If you're gonna spray spray the whole thing

this.

things look fuckey when an entire part of it doesnt have paint but the rest of the gun is painted.

Surprised to hear this. I see so many on here that are half painted.

The only ones that do that are mostly the ones that are afraid to paint, and secondary, the ones that simply replaced a receiver since the paint job.

Problem with a half paint is you do it, then realise you hate it cause its only half painted, then you finally paint the upper and they don't match right.

I say just get it over with, spray the whole thing. Maybe take off the optic if you want to but that's about it

again, this.

looks crazy to me when an upper or lower is painted and the other half isnt, or if the two are completely mismatched paint jobs lol.

either paint it all or dont.

I don’t think there’s anything wrong with not doing your rail (if it’s FDE).

Paint everything but the rail. The exposed DD FDE always looks better on a painted rig.

What I was saying on the less is more is that you don’t want to cake it on there. Just slightly dust it until you get what you want.

The attached photo is the CQBRs normal lower. The painted lower has a binary in it and I put it on there to have some fun. To each their own though.

Thanks for sharing. I like it both ways on your pics. I guess for now I will just leave it black.

Eta-elcan, peq, su233, dual switch, corncob in da wheat field.

Eta- eo 552 on the ras/ris. Arms40std,Surefire m900a- just noticed the millennium series handheld at his waste too

A little SOCOM RC blast baffle action at 700 rounds.

Noice Noice. Lets see the outside now

@Dyzastr haven't seen that top pic in a while. Never seen that second pic. Old skewl. I think CD will like that one. (552 on RAS, ARMS BUIS)

my thoughts exactly. thought of CD when I found it. usually see old school m4's set up like that. not too often do you come across a cqbr set up like that.

I've wondered what kind of affect a 3p has on overall performance with the rc?

Nice, does bring back memories

CD

The baffle wear is uneven with a 3p. Back in the day when we couldn't get 4 prongs we had an SF armorer post pics of their baffle wear and members (I think AR-Ryan may have been one of them) posted the same pics used with a SF3P. Obviously the exhaust ports don't match up on the 3P and it showed in the pics

This is pretty much current - darkening up pretty quickly.



Yeah, with the 3P two ports mostly line up and the third lands in the middle between the ports.



That's one of the reasons I wanted a 4P besides just correctness - it's really what the RC was designed for. No idea if there is any difference in wear or performance 3P vs. 4P, but I figured if I wasn't going to run a muzzle brake, I should at least have the right flash hider that aligns with the ports. Well, that and... cause 4 prong :D.

Best paint jobs come from not giving a fuck and over thinking it. Get a little "fuck it" in your system and lay down some light passes until what you like appears.

best part of it is if it looks like shit and you dont like it...spray it again.

I like my mis-matched, nothing is painted the same, setup.

The lower is actually for my M4 clone and it didn't have paint. I was just to cheap and lazy to build and SBR 2 lowers.

@Combat_Diver

Somewhat related question, might have asked you before but I can't remember.

Do you have any idea what the service life on a suppressed 10.3 barrel is? Was there any round interval they got rebarreled or just when they wouldn't group anymore? Thanks for any info.

And acetone or CitriStrip makes Krylon it's bitch and comes right off.

Lots of variables involved, don't know of anyone that keeps their suppressor on all the time.  Generally they call me if the gun won't group anymore or I stop by and gauge all their guns like I'm doing now on the road (ie barrel erosion gauge). I do try and put on new bolts at 6,000 rds.  Barrels can last longer up to 3x the amount.

CD

I do remember those pics. I know the ports don't line up with the mouse holes on a 3p, but I thought mouse holes were more of a way to relieve back pressure as the bullet exits. If that is the case, then I wouldn't think it would be too much different. I'll have to look back to see if there are any pics of dissected rc's. I would almost venture to say the mouse holes/ ports would be less eroded with a 3p as the gasses aren't directed at the holes. Unless they are, just in an off way, then I could see more erosion. Other than the blast baffle not looking symmetrical with a 4p, I'd be curious to see how the rest of the baffles look in comparison.

eta-obviously a brake is best for erosion. I would almost venture to believe the military went with the 4p over the 3p for its flash hiding benefits. I could be wrong. Just tryin to think about it from other angles. I remember the ports in the blast baffle/ first few are intended to help decrease back pressure.  So With that line of thought, the holes matching up with a 4p would just be a nice feature and not neccassarily more beneficial. I really don't know though. What say the baffle scholars?

eta- I just visualized how ports work to decrease back pressure, and now I can understand how the 4p would actually be an aid with the 4 ports matching. I was visualizing it all wrong when I first started this line of thought.

I like em dirty, clean, painted, unpainted, half painted, half unpainted....I just like em all. Can't say I've seen a cqbr I didn't like...

Looks sick!

If anyone has a bead on an SU231. Let me know!

I think you are right about they all look good.

Thank you very much

I want a purple upper reciever so bad! That looks good!

No expert on baffle design, but it seems to me that as the pressurized gas expands into the pre-blast baffle chamber area, it has five pathways into the baffle stack - through the bore or into one of the four ports on the blast baffle. Without those ports, any gas that didn't follow into the bore would be redirected as back pressure; the ports just allow some of that pressure to be relieved into the baffle stack. If so, it would stand to reason that having the gaps on the 4P aligned to the ports on the blast baffle would just help guide the gas along the path of least resistance (i.e. into one of the four ports).

Not certain that's how it actually works, but that's the way I'm featuring it.

Sounds like solid logic to me. 3 prong with RC is = to larger gas port. Eeek!

Thank god DD wisened up on their port sizing. Not that I use DD bbls on clones thus far pre URG-I.....but I am a believer in CHF so that will change once the 10.3 URG-I is released

Just saw this. Bravo!

Dude I was just wondering where you were yesterday. Am I trippin or have you been gone for like forever?

I agree. That purple looks good.

And on the topic of half painted or whatever...

I'm balls deep in the idea of painting rifles, but if I'm spending 500+ for DD rail in a specific color, It's not getting colored over lol.
Nor would I paint away my unique purple color.

Ever! Even if there's a fire!

Thanks so much! Means a lot, especially coming from someone else who knows the trials of the tanodization.

Happy 10/17 y'all!  It's my rails birthday today. And mine incidentally. My wife even baked my ris ii a birthday cake. She said it was for me but her eyes were wandering....

Ya, In my last edit this morning I came to that thought. I can understand why it AIDS in relieving back pressure, I just wasn't seeing any difference in erosion or life of suppressor/ baffles. I was more looking to see if there was uneven wear somewhere. I can see the distinct difference between the two pics, but that's just carbon build up. I'd just like to see some data on either before I'd say one causes more baffle erosion than the other is all. Not nay saying, just would like to know more.

Been gone for a hot minute, but I mostly lurk up in here anyway, lol.

Thank ya. They pop up once in a while but usually go quick.

It looks real good man! Congrats! You have any more pics?

You switched to the 416 gang didn't you? CQBR blood in.....CQBR blood out.