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MK18 Tuning Guide and Development

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Born as the CQBR, or Close Quarter Battle Receiver, the MK18 has held its weight in the ever-changing world of AR-15's. Gaining much of its popularity from its place in Military Special Operations, it became a popular firearm to clone, and a version would be sold by Daniel Defense. Easily one of the most famous short barrel rifles, especially when selecting for M4/AR-15 style variations, the MK18 stands out further for its inclusion in Video Games, Books, and Film. One of the known shortcomings of the popular rifle is that when combined with a suppressor, the added internal pressure leads to increased parts wear. Less of a problem for the infrastructure of SOF units, individual citizens exercising their right to bear arms have a series of options to remedy or reduce this problem.

mk18 optimized

We must begin by acknowledging that most American shooters rarely put enough rounds down range to wear out their firearms, this is a case of both tuning the Daniel Defense MK18 for both better performance while suppressed, and extending the lifespan of parts. An easy exception to this is competition shooters, but the MK18 is rarely considered suitable for the demands of a 3-gun match.

What is the MK18?

In its current form, the MK18 is a 10.3 inch barreled AR-15 or M4A1 with a 9.75 inch quad rail RIS II made by Daniel Defense. The military designation does not specificity a Daniel Defense lower receiver, and although there are standard issue furniture sets, these parts are often swapped out by the end user. In the end, the core of the carbine/pistol is in its barrel and rail.

As stated above, the origin of the MK18 is the CQBR, a replacement for the upper receiver of an M4A1 with a 10.3 inch barrel. The .223 Rem and 5.56 NATO rounds achieve their capabilities at velocities above 2,500 fps, which is directly tied to barrel length. The shorter the barrel the less velocity, is half the equation of a direct impingement firearm. The AR-15 requires a certain amount of pressure to reliably cycle the bolt carrier group, and the term dwell time refers to the amount of time gas pressure can build and cycle through the system as the bullet travels down the barrel past the gas port, venting pressure into the bolt carrier group.


If a MK18 is fundamentally a short barreled AR15, three elements come into play on the front end of the rifle or large frame pistol: the length of the barrel, the location of the gas port, and its location on the barrel. Too little pressure, and the bolt carrier group does not cycle, too much pressure, and the felt recoil goes up as well as the rate parts wear out. The 10.3 inch barrel of the Daniel Defense MK18 achieves an average velocity above 2,500 fps when using a standard NATO 62 grain 5.56 round (M855 Ball/green tip).

The MK18 achieves reliable cycling by opening up the gas port to compensate the short dwell time. This also translates into a harsher unlocking of the bolt, and faster rearward motion of the BCG. Contrary to 2010 video game physics (RIP MW2), attaching a suppressor does not itself slow down the projectile, but adds pressure at a rate less than inches of barrel length. The additional pressure increases the violence of the bolt carrier group movement, further complicating producing a rifle that can fire softer ammunition unsuppressed and higher pressure while suppressed without issues.

law tactical folder mk18

The MK18 achieved this, at the cost of parts wear, choosing it as the least of present evils. Far from archaic, the virtues of the firearm drive it towards urban use, where engagements are typically short and very violent.

Taming or Tuning?

Thankfully those not confined to conventional military restrictions or who are brave enough to ignore protocol have the opportunity to tune their Daniel Defense MK18 for better performance on the range, increased parts longevity, and less gas vented towards the shooter. The more susceptible parts to wear exist behind the barrel, including the bolt and bolt carrier group, extractor, and buffer spring.

mk18 JP enterprises bcg and silent capture spring system

The four/five components to be looked at are the Gas Block, Bolt and Bolt Carrier, Buffer, and Buffer Spring. First an foremost, and adjustable gas block provide the most front-end tuning, but can bring their own issues. The Superlative Arms bleed off system Adjustable Gas Block adds some wiggle room when setting the amount of gas allowed through the system, but requires a suppressor to be removed in order to make adjustments. Overall its functions in a set-it-and-forget-it format, but the included hex tool should go everywhere the rifle goes as a backup.

The Bolt and Bolt Carrier Groups have two opportunities for enhancement: coating and construction. The locking lugs take an increased beating with the higher pressure when unlocking, and the increased lubricity of Titanium Nitride coated parts make them easier to clean, but the diminishing returns lean towards a cleaner operation, and less toward decreased parts wear. Instead, the JP Enterprises enhanced bolt carrier group meets a better end-goal by focusing on operation and durability. For those with a later-model Daniel Defense MK18, the polymer dust cover will not close with the JP Bolt Carrier.

Ultimately the relationship between the gas entering the system and the pressure of the buffer spring are in contention, and exist in a balance. The JP Enterprises Silent Capture Spring Systems is a one-stop solution for both the buffer and buffer spring, and has an option for those who use the Law Tactical folder. Other solutions include Springco springs, VLTOR buffer weights, but require more attention paid to mixing and matching.

Putting the MK18 Together

The MK18 itself requires a reconniseable cost of entry for the civilian buyer, and for the casual shooter, is good to go out of the box. A simple solution for those who acknowledge their annual round count is lower than 1000, a second Daniel Defense bolt carrier group and adjustable gas block exact a smaller additional cost than tuning the whole firearm. But those wishing to push the platform to softer recoil including when suppressed and maximum longevity can turn to companies like JP Enterprises to increase overall performance. The Silent Capture Spring System does reduce the twang of the spring, but more importantly softens the full operation of the firearm cycling while reliably functioning as a whole.

JP Enterprises Enhanced Bolt Carrier Group and Silent Capture Spring System

Tuning the whole firearm by adding a Superlative Arms adjustable gas block, JP Enterprises Enhanced Bolt Carrier Group, and Silent Capture Spring System do add measurable improvements by reducing the felt recoil and effects of carbon slowing the operation of the MK18. In true Special Operations fashion, the SBR ideally hosts a suppressor which inherently introduces more carbon to the bolt. When running suppressed, there's no way to avoid the need to clean your firearms, and the enhancing the MK18 through replacing parts only adds to its longevity and performance on the range.

JP enterprises enhanced bolt carrier group

In finality, the MK18 remains a viable platform for the shorter range performance it was designed for. At the same time, the SBR and large format pistol can be improved through both continual maintenance and a progression of aftermarket improvements that tune the firearm closer to its optimal performance. For now, the MK18 is here to stay, but for those who admire the firearm, in the spirit of improvement, the project is not over.

JP FMOS Carrier Group with EnhancedBolt

MSRP: $353

Addressing Braces

At the time of this writing, Pistol Brace are under the threat of being nearly universally banned by the ATF, which doesn't have the authority of legislature. One of the casualties of the proposed brace interpretations will inherently be the MK18 pistol from Daniel Defense. Here is a full explanation of the ATF Brace Ban.

ATF Brace Ban Cover

More on SBRs, Pistol Braces, and Suppressors

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