NSWC MK18 Mod1 – What is a Mk18 Anyway?:
The Mk18 is most well known as the carbine used by Navy SEALs, though it has also been used by a number of other units and branches. Anyone who is a fan of Navy SEALs, or even a fan of military weapons in general, probably has heard of and/or seen a Mk18. But what is a Mk18, and what if I want to make a real rifle or replica rifle that matches what the SEALs use? Read on to find out what actually defines a Mk18, and what they actually look like in use.
The Mk18 grew out of the SOPMOD Block II project, and eventually became a standalone weapon system. Originally labeled the CQBR, or Close Quarters Battle Receiver, the idea was to equip shooters with a shorter barreled upper receiver to use with their existing lower receiver for use in close quarters. Naval Surface Warfare Center – Crane picked up the concept and claimed it as their own. They then developed it into a complete rifle which would be configured specifically for reliability with the short barrel. This rifle was dubbed Mk18 Mod0.
The first Mk18 rifles were cobbled together from available parts. There were only two design requirements. It needed a short barrel, and it needed a RAS. At the start, M4A1 barrels were chopped down to the front sight and the gas port was enlarged. Because the KAC RIS was in use on the SOPMOD M4A1 rifles, the same RIS was used on the MK18 Mod0. As Crane developed the rifle, they began to customize the design, adding a 5 coil extractor spring and extractor O-ring. They also developed the Crane stock, which incorporated a wider cheekpiece and integral battery storage.
It is important to note that, while NSWC/Crane was assembling the Mk18, initially they were not manufactured so much as cobbled together. They mounted the Mk18 / CQBR uppers on the receivers they had available. Many were mounted on M16A1 receivers, but many others were built using M4A1 receivers. Additionally, while the original barrels were cut down M4A1 barrels, later rifles had production barrels in both 10.3 and 10.5 inch lengths. This showcases the difficulty in defining a MK18 to specific components. Even today, there is debate regarding manufacture of barrels and upper receivers, with both Colt and LMT reportedly producing barrels and uppers for the MK18 rifles. When it comes down to it, a Mk18 is a 10.3″ or 10.5″ barreled upper with a KAC RAS or Daniel Defense Mk18 RIS II, assembled by Naval Special Warfare, and mounted to an existing lower receiver.
To both the armorers and users, the Mk18 is a short barreled carbine, most often with a rail system. The only element that is identical on every single Mk18 build is the fact that it has a short barrel. While individual rifles have occasionally had the RAS replaced with standard handguards, the other near universal component is the rail system. Early rifles had the Knight’s Armament RIS/RAS, while modern rifles have the Daniel Defense MK18 RIS II. There have also been rifles equipped with VLTOR CASV rail handguards. These rifles may also be sporting ARMS rear BUIS instead of the KAC or the older fixed rear sight.
That being said, since most Mk18s are issued as complete rifles, there is a common configuration that most off the shelf rifles today conform to. This includes not only the internal component modifications, but the LMT Crane stock, CQT Sling Mount, Daniel Defense Mk18 RIS II, KAC NT4 flash hider, and KAC 300m rear sight with a KAC front BUIS. While this is the current modern issue configuration, many previously issued rifles and uppers are equipped differently, and some newly issued rifles are modified by the user.
This is the crux of the discussion. There are Mk18 rifles as issued today, there are Mk18 variants as issued in years past, and there are Mk18 rifles configured as the user requires. Each definition of the Mk18 differs from the other. If you are building a rifle to match the Mk18 Mod1, equipped with the DD Mk18 RIS II, as issued today, you have a fairly specific set of components that should match. However, if you’re building a Mk18 as a user in the field would, you have some flexibility to change certain components, just as there is some freedom for shooters to modify the rifles at the unit level.
Photos prove that the Mk18 rifles in use have been equipped with a number of different aftermarket components. These include, but are not limited to:
BE Meyers Flash Hider
Surefire Flash Hider
A2 Flash Hider
Magpul ASAP Sling Mount
KAC 600m BUIS
LMT Rear Sight
VLTOR Mod Stock
M4 style Stock
When it comes down to it, if you take a 2004 Ford Mustang, and put Eibach rims and an aftermarket exhaust system, it’s still a 2004 Ford Mustang. With the Mk18, many users in many different groups have modified their rifles to fit themselves and their applications. Depending on the look you are going for, there are a number of different components that have been used by someone at some point on a Mk18.
In the end, you need two things. You must have a short barrel, either 10.3 or 10.5 inches. If it has a longer barrel, it’s a SOPMOD M4. You should also have a rail system, usually the KAC RIS or DD RIS II. As long as you stick with components that are or were in use by someone, somewhere, you’ll end up with a rifle that is conceivable, even it it isn’t 100% off-the-shelf stock.
This picture shows a Knight’s Armament DMR equipped with an ERGO grip. While the customized rifle isn’t a Mk18, it shows that there is some level of modification being done on rifles.
This photo of a Special Boat Services Crewman shows a Mk18 with a Meprolight sight, an old style M4A1 stock, and what might be a KAC 600m BUIS mounted backwards.
This photo shows a member of a SEAL Sniper Team with a Mk18. He has an unidentified vertgrip, and has the 300m KAC BUIS mounted in front of his ACOG.
A member of Special Forces with a Mk18 and an old style M4 stock.
A Navy SEAL carries a Mk18 equipped with an ACOG and a VLTOR Modstock
In the photo below, one Mk18 has a Tango Down BattleGrip, one on the far left of the photo has a folding Mako style vertical grip. The shooter kneeling in the center has a Mk18 with a Magpul stock, either a CTR or MOE.
Navy SEAL (as shown in above photos) with a VLTOR Modstock on his MK18.
The Mk18 shown below has a Magpul MOE stock on it.
The Mk18 in the picture below is equipped with an M4A1 lower and an unidentified suppressor.
I’ll leave this one to you. The picture was marked ST10, but the rifle is different than most I’ve seen. It’s equipped with a PRI flip down front sight gas block, and an ARMS rear sight. A compact reflex sight is mounted on a QD riser, but the rifle and rail are unidentified. Is this a SEAL from Team 10, and what rifle is it?
Disclosure: Every picture is correctly labeled to the best of my knowledge. I have not vetted the source of the photos, or attempted to prove their validity beyond original labels. Knight’s Armament Corp makes both a RIS and a RAS. One has numbers that count up, one has numbers that count down. The KAC RAS has a newer mounting system. I may use RIS and RAS interchangeably.