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Discussion Starter #1 (Edited)
Hi everyone,

I recently documented the removal of the stock muzzle devices on both the factory 16" and 13" CQC barrel assemblies for the SCAR 17S. Lots of good info and tips in there if you plan to tackle this project yourself, but if you go to page 2 of that thread and read from there, you will see just how much trouble I've gone through up to this point to find a way to get this KAC suppressor mounted - I had no idea it would be anything but plug and play. Good news is, with a slight modification from your local machine shop, it basically is a plug and play setup. More below.

This thread will be a separate tutorial focusing on the installation of the Knights Armament 7.62 QDC flash hider (procedure is same for the QDC MAMS brake) that is required to mount a KAC 7.62 QDC suppressor.

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First thing's first - take your QDC device to your favorite machine shop! This is what you need to have them do to it:

The instructions are in the captions within the picture below, but here are a couple of supplementary notes and details:

1. The caption in the pic below says you only need to machine 1/8" deep into the opening of the QDC, which is true. However, you might've noticed from the picture that my machinist wasn't listening that well (it was noisy and I was jabbering away to the guy while he was setting up the lathe... haha!) and he went a bit deeper than that. He actually took off like 1/2 of the 1st thread and there is still a burr where that thread was cut. I need to find a way to remove this burr somehow before I can actually thread it on to the 17S barrel to test fit. Darn! Still have to wait...

I will likely take it back tomorrow to have him clean up the burr since I don't have any kind of file or tool to do it at home. He actually offered to clean it up right there on the spot, but I said, "Nah, I'm sure I can find a way to take care of that at home." Oops. Now I wish I took him up on it. I'm just very glad it wasn't my $400 (yes $400!!!) QDC MAMS brake that he took some thread off of. Believe me - I'll be extra careful to make sure he doesn't goof up on that one when I bring it to him! I don't think having ~1/2 of the beginning of the 1st thread missing will affect anything, but still, there's no need to touch the threads at all for this installation.

The good news is he knocked it out of the park besides that. With my Mitutoyo digital calipers, I'm measuring the new opening diameter in the QDC device @ 0.6880". The stock FN flash hider is 0.6820" so between my conservative math and his conservative cutting, we ended up at 0.006" larger diameter than the stock FN flash hider. I'd say that's about as perfect as I could ever ask for! It will definitely float over the 17S barrel shoulder/diameter, which is the goal here. Long story short, stick to 1/8" deep and the lathe bit won't ever touch your threads and your muzzle will still bottom out so you can time the QDC off of the muzzle!



2.
It is not likely that my QDC device will time correctly without any timing shims, so I am working with Knights Armament currently to see if they can manufacture a shim kit for this application. You can see the "alignment pin" (that little nub) sticking out of the QDC device towards the bottom right in the picture below - according to KAC, this needs to be oriented at 12 o'clock once installed to be timed correctly.

FYI - The alignment pin (shocker!) aligns with a notch in the QDC suppressor's locking collar before you can ratchet it down to secure the suppressor on the QDC device. I'm not so sure that it REALLY needs to be at 12 o'clock for anything other than aesthetic purposes, but in case it has to do w/ POI shift and how they design their QDC devices to expel the hot gas, I'm not going to take any chances if I can be patient and wait for a proper shim kit to be made by either KAC or a 3rd party manufacturer.

Even if I get super lucky and it ends up right at 12 o'clock, someone else besides me will eventually run into this same issue. It's probably in KAC's best interest to just make a shim kit that can drop into this 5/8" x 24 QDC device. That is already how their shims work for the 3/4" x 24 QDC, and it would be needed for any installation where the 5/8" x 24 QDC has to be timed off of the muzzle instead of a shoulder - this likely isn't just a SCAR 17S limitation. If KAC wants their suppressors on more rifles, they should just make the kit already!






***Update*** 1 day later

Success!! My machine shop cleaned up that burr earlier today and the QDC fits like a glove. Words cannot describe how satisfying this is.





Here is the QDC muzzle device nearly to the shoulder of the SCAR 17S barrel.





... and here it is installed, floating perfectly over the barrel and seated fully against the muzzle!





***Now just waiting on a KAC to respond to an email to let me know whether or not they will be making the necessary shims to finish this installation. God, I hope so.***

***After I apply the Rocksett thread adhesive and do a final torque down of the muzzle device, I will be using a
Geissele suppressor alignment rod to check for what will hopefully be perfect alignment between the suppressor/QDC flash hider and the bore.***

Stay tuned!

 

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Discussion Starter #4
FYI, that muzzle device permanently attached on the ~13" barrel is 16.1"
Interesting! Too late for me though since I Form 1’ed the 17S years ago. Good info for others who might be trying to avoid the SBR process. I haven’t received my MAMS brake yet, but I’m curious now if the MAMS would get you to the same 16.1”...
 

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Discussion Starter #5 (Edited)
Update:

I've been working with Knights Armament and their OEM shim supplier to have this custom shim kit made, so everything is lined up for a production run. Just waiting on a critical measurement from someone at Knights to finalize OD/ID dimensions of the shims. If anyone out there is planning to install a 7.62 QDC suppressor on your SCAR and need a shim kit to time it,
please feel free to shoot me a PM or email [email protected] (remove "SPAM" first) if you want to be notified once they are available.

Thank you!
 
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