SILENCER SATURDAY #236: Knight's Armament 556 QDC CQB – Full Auto KAC SR-15

by Pete
SILENCER SATURDAY #236: Knight’s Armament 556 QDC CQB – Full Auto KAC SR-15

Good afternoon everyone and welcome back to TFB’s Silencer Saturday, brought to you by Yankee Hill Machine, manufacturers of the YHM Turbo K rifle suppressor. Last week we brought you the Knight’s Armament KAC 7.62 QDC legacy rifle suppressor. This week we return to the world of Knight’s with a look at the KAC 5.56 QDC/CQB rifle suppressor on both the 11.5″ carbine length gas and the intermediate length gas KAC SR-15.

SILENCER SATURDAY #236: Knight's Armament 556 QDC CQB - Full Auto KAC SR-15
Parts List:
Special Thanks: Freedom Trading Co.

If you are a fan of top-tier long guns and upper receivers, you know that trying to acquire anything from Knight’s Armament is on par with finding a diamond in your backyard. The good people at Freedom Trading Co went above and beyond great customer service to get me a 14.5″ SR-15 upper receiver group for this review (no discounts or anything, they just saved my place in line). If you are a fan of Silencer Saturday, please support the dealers that support us. Follow them on Facebook and Reddit for information on the next KAC drops.

SILENCER SATURDAY #236: Knight’s Armament 556 QDC CQB – KAC SR-15

SILENCER SATURDAY #236: Knight's Armament 556 QDC CQB - Full Auto KAC SR-15

As the saying goes, life is all about balance. Too much or too little of a good thing and everything around you will start to make adjustments. In the world of firearms, especially semiautomatic carbines and rifles, the balance between suppressor length and barrel length is an important consideration. On one end of the spectrum, a long barrel with a rifle length gas system paired with a long suppressor may provide the quietest report and the smoothest recoil impulse. But unless you are strictly a benchrest shooter, a combined barrel/suppressor length of ~30″ is impractical.

On the other hand, a short barrel with a carbine length gas system and a short K-sized suppressor has the potential of being both loud and gassy for the shooter. In the middle of these scenarios is a delicate calculation of a shorter barrel with a longer suppressor to arrive at the sweet spot between quiet and compact. The length of the gas system, the internal geometry of the suppressor, the barrel’s gas port size, and to a lesser extent the recoil buffer weight, all play a combined role in the overall system’s performance.

The 5.56x45mm cartridge was initially designed for 20″ barrels with rifle length gas systems. As mission requirements changed, barrels became shorter, gas system lengths became shorter, and dwell times adjusted. I am neither an historian nor an engineer, so the full details on the development of the AR-15 gas system should be left to someone more qualified (such as TFB’s very own Matt Moss).

SILENCER SATURDAY #236: Knight's Armament 556 QDC CQB - Full Auto KAC SR-15

From a laypersons point of view moving the gas system farther down the barrel allows for a more complete burn of powder, reducing the amount of gas a debris that can be forced down the gas tube, the barrel, or into a suppressor. Of course as barrels get shorter, the gas system must be reduced (it is impossible to run a rifle length gas system on a MK18 for example. And even if there is enough barrel real estate for a longer gas system, there still needs to be enough distance between the gas port and the muzzle to provide enough dwell time to cycle the action.

What are the gas system lengths of some of the most common AR-15 models and common barrel lengths? Let’s take a look at a chart that details six of the most popular combinations.


Barrel Length (Inches)

Gas System

Gas System Length (Inches)













Geissele URG-I




KAC 14.5








As you can see from the above chart, the M4 and M16 have the longest gas port to muzzle lengths and the MK18 followed closely by the KAC 14.5” have the shortest span between the gas port and the muzzle. As an interesting aside, NSWC-Crane testing showed that a M4A1 mid-length gas system had rate of fire and lifespan performance benefits over the standard M4 carbine gas system.

SILENCER SATURDAY #236: Knight's Armament 556 QDC CQB - Full Auto KAC SR-15

Today’s review is of the KAC 556 QDC CQB suppressor which has a total length of five inches, including the mount. In my opinion, this is a K-sized suppressor that is built to optimize length over maximum sound reduction performance. Is it better suited for the KAC SR-15 CQB 11.5″ carbine gas system or the KAC SR-15 14.5″ intermediate gas system? The answer is yes.

First, let’s take a look at the numbers.

KAC SR-15 14.5″ Upper Receiver Group
KAC SR-15 11.5″ Upper Receiver Group
KAC 5.56 QDC/CQB Suppressor

The mission dictates the gear*. It sounds like the line from an action movie, but it is one of the most important factors in procuring guns and gun parts and one of the most overlooked considerations. While three inches of barrel length is not a huge difference, the reality is that extra time the bullet spends gaining velocity can make all the difference in terminal performance.

* Your mission can be purely fun and entertainment. It doesn’t have to be about fighting enemies.

SILENCER SATURDAY #236: Knight's Armament 556 QDC CQB - Full Auto KAC SR-15

ABOVE: A reminder for those of you who want to swap out muzzle devices on your SR-15. Use the KAC Knight Stick barrel extension wrench and not an AR-15 barrel extension wrench. The lugs on the KAC E3 bolt are rounded. Or send the upper off to a certified KAC armorer.

Ammunition, especially bullet designs, are optimized to react in tissue at certain velocities. Besides making incredible fireballs, one of the reasons there are not many 5.56x45mm barrels shorter than 10.3 inches is that velocities at usable engagement distances are not adequate for expansion, tumbling, and wounding channels. Again, I’m not an expert in this area and there have been multiple books written on this exact subject.

The fact remains that if your mission is defense, added barrel length is important, up until the point where your weapon becomes unwieldy. In my opinion, when choosing to suppress supersonic rifle rounds, I would rather choose a shorter silencer paired with a longer barrel versus a longer silencer with a shorter barrel.

As expected the KAC 5.56 QDC/CQB on the 14.5″ upper with the Knight’s proprietary intermediate gas system is objectively quieter than the 11.5″ with the carbine gas system. Again, in my opinion, the KAC 14.5″ upper with the KAC 5.56 QDC/CQB is as close to the perfect “fighting rifle” as you can get, balancing length, suppression and muzzle velocity. If you are ok with sacrificing velocity, the 11.5” SR-15 paired with a KAC 556 QDC or a SureFire SOCOM RC2 iOS a solid setup.

A couple of notes on the above video. I forgot to swap buffers between the 11.5” and the14.5” strings of fire. And these are the first rounds through the 14.5” upper; there may be some oil or protectant burning off the barrel. Big thanks to my boys and silencer models Luke C. and Hop.

Thanks for reading. Be safe, have fun, and we’ll see you next weekend for another Silencer Saturday.

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  • TFB TFB on Jul 20, 2022

    silencer Saturday hmm I just got this issue on Wednesday

  • Gary Gary on Aug 03, 2022

    When will the results of this review be added to the spreadsheet on your website?