The Cobalt Kinetics Muzzle Device: 2.75 inches

The RCB-KM muzzle device from Cobalt Kinetics allows for a very low recoiling gun but more importantly, an overall length of 16 inches from a 13.7 inch build. The RCB-KM will also accept a deadair silencer. Perfect for close range matches!

If you’re thinking about building a 13.7 inch barreled rifle for close range rifle work such as competitions or training, this is the article for you.

First, let’s do some quick math. If I have a 13.7 inch barrel, what do I need to permanently attach to make an overall length of 16 inches? Good work, at least a 2.3 inch muzzle device. Which as you may know, there isn’t a crazy large amount of these on the market.

Note: I did find a list of over 2.3″ devices that a company graciously compiled back in 2021. Thank you

Add Cobalt Kinetics to that list. The RCB-KM Deadair Keymo muzzle device eats recoil and allows for a deadair silencer to be attached. It is also 2.75″ long allowing for a 13.7 inch barrel to reach an overall length of 16 inches.

gun build, vltor a5, cobalt kinetics muzzle
This is a 13.7 inch build, now with the Cobalt Kinetics muzzle device it reaches an overall length of 16 inches. Notice the VLTOR A5 buffer system. Pair this with the muzzle device and there is virtually no recoil.

The Muzzle Device

Price: $107.76

  • Length: 2.75″
  • Weight: 6.2 oz
  • Material: 4140 Steel
  • Coating: Nitride
  • RC Hardness: 32-34 RC
  • Tread pitch: 1/2X28 tpi & 5/8X28 tpi
  • Calibers: 5.56 X 45mm (NATO) & .30 Cal


It was pretty easy to install. Shims did come with the device and each shim size was nicely labeled and separated out.

midwest industries armorers wrench
The RCB-KM deadair muzzle device has flats that will accept an armorers wrench for easy install. This is a midwest industries armorers wrench.

Now pin and welding, that’s a different story.

To make an overall barrel length of 16 inches from a 13.7 inch barrel, there will need to be a permanent modification done, i.e. a pin and welded muzzle device to the barrel. There are a few things to take into account when getting a pin and weld done.

  • You don’t have to use your “local gunsmith”. There are a multitude of companies that you can purchase a pin and weld service online from that are very very good. Simply box up your upper in a non firearm looking box and send it off to the company. Easy and legal. One of these being Dwilsonmfg.
  • If the pin and weld is done poorly or without the proper tools it can damage your muzzle device and your barrel. The drill bit should just barely go into the barrel. Poor pin and welds will go too deep into the barrel damaging the barrel.
  • The weld is important. If the weld is done poorly it will be hard to remove the device if you ever want to replace it.
  • A hand drill is not a proper tool if you’re paying for a service. The drilling should be done with a mill.
pin and weld
This pin and weld was completed by Dwilsonmfg. Notice that the weld is very clean. Don’t forget to function check the gun with muzzle device installed properly before sending it off.

The Recoil Build

The goal of a 13.7 inch rifle is to be able to move quickly from target to target whether that is running or just shifting the gun. To do this, not only does the length and weight matter, but also the recoil system. The RCB-KM eats recoil. Now pair that with a Vltor A5 recoil system and you have a very flat shooting gun. This allows for faster follow up shots and finding your sight picture faster.


So where do these shorter builds shine? Well, in matches that involve quick target engagements and even in some long range gas gun matches.

  • 2gun: Many local clubs will run 2 gun matches. Often a non-sanctioned club match, these matches involve close range engagements with both a pistol and a rifle. There may be some 200-300 yard engagements however many are running red dots on their rifles so the vast majority of the engagements are inside 25 yds. Speed is the goal here.
  • RPGI night matches: These day-night matches directed by RPGI have blown up. With a two on paper scoring scheme and fast close range targets, a shorter carbine fits the bill. Don’t forget the white light! Keep an eye out for these matches. It’s not shocking that 150 shooters want to show up to a match that has physical challenges, 7 stages in the day, and then 7 more at night. NVG or white light is allowed.
  • Precision Gas Gun Matches: Yes, even a short gun can do work at long range gas gun matches. There are a few options for shooting long range matches such as under Quantified Performance or within the IWI Gas Gun Challenge.
rpgi night match shooter
This picture was taken at the last RPGI day nite match in West Virginia. Small carbines are great to run at these matches. This shooter was using night vision
Anna Martinez
Anna is a Federal Weapons Gunsmith with 8 years of previous experience within the US Army Ordnance Corps (91F). She has taken multiple armorer and weapons proficiency classes to include FN, Knights Armament, and Small Arms Weapons Expert course. She also writes for American Gunsmith, AR Build Junkie, and is active in Precision Rifle sports around the nation.