The MK18 is a 10.3 inch short barreled AR-15 made by Daniel Defense. It’s grown in popularity in large part due to its utilization by Special Operators wielding it in the global war on terror. It will do most anything your typical AR-15 can do but better – with a couple of caveats.
When you have a barrel as short as the MK18’s you run into some issues. Mainly dwell time…
“Dwell time is the time that your gas operated weapon maintains pressure to continue the cycling of the weapon. It primarily exists from the time the bullet passes the gas port in the barrel to the time the bullet exits the muzzle.” – (www.ar15.com)
The dwell time for the MK18 is horrifically short, to rectify this Daniel Defense opened up the gas port so that the gun is more forgiving of ammunition types like more weakly powered ammunition such as Tula .223. The problem is that if you are using full powdered 5.56 you’ll have an excess of gas being spit back into your face around the charging handle and the gas port. It isn’t bad but it’s certainly something that is different from your standard 14.5 or 16 inch AR-15. The upside is that this rifle will run with any ammo and reliability will never be a concern with your MK18.
If you want to fine tune your gas system you have several options available. You can install a gas block which allows you to change the amount of gas used to cycle your rifle, such as the excellent MicroMoa Govnah. Or you can go with a heavier buffer weight or stiffer spring. On my personal MK18 that I use I have opted for the VLTOR A5 recoil system which in a nutshell is a rifle length spring with a variety of buffer weights to suit your needs. The rifle length spring lowers the velocity on the bolt carrier group allowing for less perceived recoil. I also have a very heavy buffer for the A5 system weighing in at 6.83oz (A5H4) since I primarily run the rifle with a Surefire SOCOM RC suppressor, which significantly increases backpressure. Besides using the above mentioned you can also install charging handles which are meant to stop the flow of gas from reaching your face. One excellent product that I’ve heard of but I’ve yet to use is the PRI gasbuster. Regardless of the method you choose or combination thereof if you’re planning on running a suppressor on the MK18 consistently I would recommend having some type of gas mitigation system.
The upside to the short 10.3 inch barrel on the MK18 is how maneuverable and balanced it feels. Easy to manipulate in and around tight corners and indoors this is the perfect gun for short range work. While you do have loss in the speed of the 5.56 slug leaving the barrel, you also have more than enough to reach out and touch targets out to 300m with terminal efficiency as long as your ammo choice is sound. If you regularly push your AR past 300m and hope for excellent wounding ballistics the MK18 is not your gun. It isn’t a question of accuracy but rather bullet speed. It’s definitely an assaulter’s weapon and it excels in this role. Another great part of rocking a short barrel like the MK18 is even with a suppressor attached you still have a compact package. In my case I use a Surefire SOCOM RC which has a total length of 6.2 inches putting my total barrel length at 16.5 inches in length which is just over the length of many standard AR-15 makes and models you see out at the range today.
The MK18 has more recoil than your traditional AR-15 however it’s nothing that isn’t manageable or where the trade-off vs compactness isn’t worth it. The MK18 features a cold hammer forged barrel and accuracy is spectacular. Shot groups using a Trijicon TA31RCO 4×32 optic and reloads utilizing 62 grain bullets produced groupings ranging from 1.27 inches to 1.05 inches at 100 yards. This rifle is likely more accurate than you and accuracy out to 300m is my main concern. My groupings at 300 meters averaged between 4-5 inches and I fully believe I could consistently achieve somewhere around 3 inches with a higher objective magnification.
The MK18 sports a Rail Interface System II (RIS II) handguard. It’s no lightweight at 14.1 oz and it isn’t small like many of your modern MLOK and Keymod rails. However due to the rail being so short the rifle itself does feel very well balanced. Another note is that the handguard is tough as nails. The handguard has been known to take extreme amounts of force without bending. I have utmost faith that this rail can take any of the rigors I can throw at it and it would make it out unscathed. The stock that comes with the Daniel Defense MK18 was good, however I’m personally more partial to the Bravo Company USA gunfighter stock. The trigger that comes with the Daniel Defense is a good solid milspec trigger with a clean break. However I once again swapped it out for a Geissele G2S trigger which has become my go-to on my AR-15’s. I’m not against the buttstock or trigger but rather my preferences as a shooter led me to choose other accessories.
To date I’ve fired approximately 3,500 rounds of Freedom Munitions 5.56 through this rifle and I’ve been extremely pleased with what I’ve seen thus far. I’ll be the first to say, that if you don’t plan on running a suppressor primarily then a 11.5 inch barrel length might be more suitable for you. An 11.5 barrel has a significantly increased dwell time over the 10.3 inch barrel. However if you’re looking for the shortest you can get with the AR-15 while still maintaining excellent reliability then the MK18 is an excellent option. The MK18 has some recoil and gas blowback issues. However it balances like a dream, runs reliably no matter the conditions, is handy and maneuverable and will quickly become your favorite rifle.