Colt / Arms Unlimited MK 12 HBAR LE Rifle History


In the weeks following the tragic October 2017 Las Vegas shooting, a metropolitan police department was looking for a marksman rifle and/or upper receiver assembly that could be used to deliver accurate fire in an active shooter situation out past average carbine engagement ranges.

Law enforcement responding to the active shooting event in Las Vegas 2017.
Notice the mix of shotguns, handguns and one carbine.
Photo courtesy of the New York Times

The department was looking at fielding a stand-alone rifle or a rifle with both a carbine upper receiver and an improved capability (rifle) upper receiver. The department wanted their officers to be able to switch upper receivers as tactical needs changed. The department requested an 18” barrel because they felt that an 18” barrel was optimal for their rifle deployment policy. Arms Unlimited (AU) in Las Vegas, NV does a significant amount of law enforcement sales. The police department approached AU with their specifications. Arms Unlimited then developed the ‘MK 12 HBAR’ in conjunction with personnel from Colt’s law enforcement training division. Testing of the rifles was conducted in Las Vegas, NV and Hartford, CT.


The rifle being named “MK 12 HBAR” created controversy immediately due to the prior history of the MK 12 Mod. 0, MK 12 Mod. H, and MK 12 Mod. 1 rifle’s that had been used by the United States Army, Navy and Marine Corps in military operations around the world. Many people in the firearms community, especially the US military weapon cloning community, felt that the use of the MK 12 designation was misleading and disrespectful to the official military rifles. AU was never intending to ride on the reputation of the MK 12 series of military rifles. The name simply evolved from the initial conversation with the requesting department when they stated “we want something like a MK 12 platform”. The name ‘MK 12’ just stuck after that initial conversation. Additionally, the rifle shared a couple of similar external features of the MK 12 Mod. 1 (fixed stock and Knights Armament free float RAS assembly). It is probably safe to say that AU will put more thought into a nomenclature in the future if the situation presents itself again. I’m not going to get hung up on the name.


The foundation of the AU MK 12 HBAR is the Colt model LE6921CHP rifle. The LE6921CHP rifle began life as a California Highway Patrol specification rifle which they have since stopped using. However, three other California law enforcement agencies still use the rifles so Colt didn’t change the model number. In the factory configuration, it is essentially a Colt LE6920 with a 14.5” M4 profile barrel and a rifle length stock. These rifles were initially classified as Short Barreled Rifles (SBR) but were removed from the NFA registry by AU when the lower receivers were used as the foundation of the MK 12 HBAR rifle project. The LE6921CHP’s that were used for the MK 12 project had only shipped with a black sling per original product specifications. Current LE6921CHP’s ship with a 30-round PMAG, sling and manual.


Maintaining use of a rifle length stock was important in the requesting police department specifications because carbine stocks were not compatible with existing rifle locks in the department’s vehicles.

The stock configuration used in the LE6921CHP has an insert in the rifle length receiver extension to enable police departments to use a carbine buffer/action spring operating system in an A2 rifle stock. The advantage of this is that the lower receiver configuration can run both a carbine length gas system upper receiver or a rifle length gas system upper receiver.

This image shows the layout of the MK 12 operating system.
‘H’ Buffer
Another view of the ‘H’ buffer
Carbine length action spring
Front view of the spacer that goes in the rifle length receiver extension. This spacer occupies the space that is normally occupied by a rifle length action spring. Also shown is the rifle stock screw.
Back side of rifle length receiver extension plug and head of the butt stock screw

Testing showed that a rifle length operating system had problems reliably running a carbine length upper receiver. An additional benefit to the department using a carbine length operating system is that it enabled them to use various weight buffers (H, H2, etc.) as needed for various rifle accessory applications.

Knights Armament Rail System

Knights Armament SR-15/16 Free Floating Rail Adapter System PN: 99167

The barrel is free floated due to the use of the Knights Armament SR-15/16 Free Floating Rail Adapter System PN: 99167 which is similar to the Rail Adapter System (RAS) used on the military MK 12. Mod. 1 rifle. The RAS includes three 11-rib rail covers as provided from Knights Armament.

Top view of the KAC rail system


The MK 12 has a rifle length gas system with a pinned low-profile gas block. The Colt 1/9 HBAR barrel, manufactured by Wilson and non-chrome lined, was selected in the department specifications as a result of its demonstrated accuracy with 55 gr to 69 gr ammunition used by the department.

The Colt / AU MK 12 has a rifle length gas system
Bottom of the low profile gas block

The barrel is threaded 1/2 – 28 TPI and has a standard A2 flash hider installed. The barrels originated as 24” barrels that were used in Colt’s previous Accurized Rifle product line. I had received a couple of questions as to why the MK 12 barrel only has ‘HBAR’ stamped on it and not ‘HBAR ELITE’ like the 24” and 20” rifles of the Accurized Rifle series. Colt only put ‘ELITE’ on barrels that had a target crown. The barrels used in the MK 12 project, as previously mentioned, had flash hiders installed. So, ‘ELITE’ was not stamped on the barrels.

Barrel markings shown are C MP 5.56 NATO 1/9 HBAR. Notice the top of the low profile gas block.
Business end of the rifle barrel
Side view of the A2 flash hider, barrel and gas block


The lower receiver assembly is a standard (with the exception of the rifle length stock discussed earlier) ‘M4 Carbine’ marked lower receiver as used on the LE6921CHP rifles. The lower receiver serial numbers are from the ‘CR’ serial number range and has a small QR code on the magazine release recess on the left side. The manufacturer markings by the selector on the left side are ‘Colt Defense, Hartford, Conn, U.S.A.

The lower receiver is also equipped with an ambidextrous selector and standard A2 grip. The rifle shipped as a semi-auto only rifle with two selector positions…safe and fire. The trigger group is a standard mil-spec trigger group.


The upper receiver is a flat-top upper receiver. It does not have ‘M4’ on the front of the 1913 rail as we typically see on M4-type upper receivers. Additionally, there are no T-marks in the rail recesses.

On the right side of the upper receiver there is a ‘C’ and a Cerro Forge ‘keyhole’ mark. These external upper receiver features are consistent with the upper receiver on my CR6720 HBAR Elite rifle from the Colt Accurized Rifle series.

C (indicates Colt upper receiver) and Cerro Forge ‘key hole’ forge mark

The charging handle is a standard mil-spec charging handle.

The bolt carrier group is a mil-spec M16 bolt carrier group. There is not a ‘C’ on the bolt carrier. The bolt is ‘MP’ marked.

In this view you can see the MP stamp and the improved extractor components. The improved extractor has the addition of the black o-ring around the extractor spring and the black plastic insert inside the extractor spring.


There were not any back-up iron sights provided with these rifles as the requesting department had on-hand iron sights that they were going to install themselves along with optics.


As often happens with law enforcement acquisition programs, budget changes caused the police department purchase plan to stop. Arms Unlimited had provided support for the project upfront and were left with the 19 completed rifles and 28 upper receivers that had been assembled. As a result of Arms Unlimited providing on-hand LE6921CHP rifles and providing advance funding for the upper receiver assemblies made by Colt they wanted to recover their investment for these rifles and upper receivers. They made the upper receivers available to the civilian firearms market for purchase in July of 2018. The rifles became available a short time after that. Of the 19 rifles, one rifle was kept by the owner of AU leaving 18 rifles available for public purchase. The Colt sales panic of late September/October 2019 caused sales of the remaining inventory to surge.


Standard Colt AR-15 Shipping Box

The rifles shipped to the buyer in the Colt factory LE6921CHP boxes that contained the rifles which the MK 12 lower receivers came from. Unfortunately, the end labels were removed. I believe the labels were removed simply because the label no longer reflected what was actually in the box. I have ordered a replacement label from Colt that will have the original factory label data on it.

The internal box serial number label with QR code remained so you could still extract some data with a QR code reader app. This data contained in the QR code is limited to the model number, serial number and the date that assembly of the rifle began in the factory.


My cost for the rifle to my FFL was $1495.00. My FFL transfer fee was $25.00. Total price $1520.00

Final Thoughts

Overall, I am very happy with the rifle. The fit and finish is first class with no defects. I really like the contrast of the heavy barrel with the black parts of the rifle. As a collector, I always keep my Colt rifles in original configuration. However, this rifle will be my one exception. I will order a set of Knights Armament back-up iron sights for it. I will put a magnified optic of some type on it as well. I need to shop around and see what I can find. I look forward to taking the rifle to the range to see what it can do.


Weight of lower receiver assembly: 2 lbs 10 oz

Weight of upper receiver assembly: 5 lbs 13 oz

Total weight of assembled rifle: 8 lbs 7 oz.


Visible inside the grip is F2
Detail view of selector. Note the S
Detailed view of the selector. Note the ‘dot’
Detail of the right side of the ambidextrous selector. Note the 4 that is visible in the lever arm.