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Discussion Starter #1
Hi everyone,

I'm about to run my SCAR 17S SBR suppressed for the 1st time, and I'm in the middle of swapping out the gas control screw in my factory 13" barrel assembly for a 1.85mm factory screw, which is supposed to be the correct size in a "real" Mk17 CQC. However, after removing the stock gas control screw, I found that it's actually smaller than the 1.85mm one I purchased from Midwestgunworks.com. The stock one looks to be about ~1.60mm.

Why would the real Mk17 CQC have a larger gas jet than the one in the 13" barrel assembly that civilians can buy? From what I had read about the topic previously, I was under the impression that the civilian gas jets were larger because civilians "use crap ammo" and it was to compensate for under-powered target ammo. Or are the civilian gas jets actually smaller to keep from blowing up guns w/ overpowered ammo?

I'm just wondering if I should have left the factory ~1.60mm screw alone and gave it a try first since that would beat the gun up less in either gas setting...

Has anyone else out there experimented with different gas control screws in their suppressed SCAR SBR w/ factory 13" barrel? Would really like to hear what you've learned.

Thanks in advance for any input or advice you might be able to offer!
 

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Discussion Starter #2
Also, are these gas control screws supposed to have Rocksett adhesive on them? The stock one that I took out didn't appear to have any Rocksett residue or anything. It just broke free as expected with a firm twist of my Wera 4mm slotted/flat head screwdriver. Now, I'm sitting here wondering how these gas control screws don't come loose from all the heat and vibration...
 

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Also, are these gas control screws supposed to have Rocksett adhesive on them? The stock one that I took out didn't appear to have any Rocksett residue or anything. It just broke free as expected with a firm twist of my Wera 4mm slotted/flat head screwdriver. Now, I'm sitting here wondering how these gas control screws don't come loose from all the heat and vibration...
don't use anything on the gas jet. It will stay in place by itself. If anything some people add a slight amount of anti-seize to them.
 

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Hi everyone,

I'm about to run my SCAR 17S SBR suppressed for the 1st time, and I'm in the middle of swapping out the gas control screw in my factory 13" barrel assembly for a 1.85mm factory screw, which is supposed to be the correct size in a "real" Mk17 CQC. However, after removing the stock gas control screw, I found that it's actually smaller than the 1.85mm one I purchased from Midwestgunworks.com. The stock one looks to be about ~1.60mm.

Why would the real Mk17 CQC have a larger gas jet than the one in the 13" barrel assembly that civilians can buy? From what I had read about the topic previously, I was under the impression that the civilian gas jets were larger because civilians "use crap ammo" and it was to compensate for under-powered target ammo. Or are the civilian gas jets actually smaller to keep from blowing up guns w/ overpowered ammo?

I'm just wondering if I should have left the factory ~1.60mm screw alone and gave it a try first since that would beat the gun up less in either gas setting...

Has anyone else out there experimented with different gas control screws in their suppressed SCAR SBR w/ factory 13" barrel? Would really like to hear what you've learned.

Thanks in advance for any input or advice you might be able to offer!
Regarding gas jet size in the 13" barrel, check out this thread on AR15.com


There are a lot of responses for various barrel lengths of the SCAR 17 and 16.
 

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Discussion Starter #6 (Edited)
Thanks! Although it doesn't necessarily answer my question, the following response from that thread was pretty helpful:

"SCAR 17 with factory 13" barrel, Surefire SOCOM 762 Mini 2

M118LR, set to suppressed
1.55 mm does not lock back on last round.
1.60 mm (stock gas jet) cycles perfectly. Fairly soft, minimal gas-to-face.

Magtech CBC 168gr, set to suppressed
1.40 mm does not lock back on last round.
1.45 mm locks back on last round, but the bolt feels extremely slow, as if it just barely touches the end plate in the receiver.

I am sticking with the stock gas jet. If you have a higher back pressure can you can probably use a smaller jet, though."


I ended up installing the 1.50mm gas screw (stock was 1.60mm) because I anticipate a bit more back pressure than what was likely experienced above with the Surefire SOCOM Mini. I can always go up in size if I experience any failures due to a lack of gas. Would rather do this than risk over-gassing from the start with a larger port. I also did NOT use any Rocksett, thankfully. Sounds like that would have been a major no-no! The new screw was easy to set in there, and it was obvious when it was tight enough.

I am still curious why the official Mk 17 CQC spec called for the 1.85mm gas screw... seems like that would be majorly over-gassed, no?
 

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Maybe the stock military rifle isn't going to be using commercial 308 ammunition which is higher pressure. That means NATO only needs the larger gas port to run in all conditions? Just a theory.

My 17 SBR runs with a 1.45 with NATO surplus M80 in the 12 o'clock position and runs perfect with a Phantom M2 QD in the 10 o'clock position. The 1.45 was just enough for full operation but I might go to a 1.5 just for peace of mind.
 

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Maybe the stock military rifle isn't going to be using commercial 308 ammunition which is higher pressure. That means NATO only needs the larger gas port to run in all conditions? Just a theory.

My 17 SBR runs with a 1.45 with NATO surplus M80 in the 12 o'clock position and runs perfect with a Phantom M2 QD in the 10 o'clock position. The 1.45 was just enough for full operation but I might go to a 1.5 just for peace of mind.
Which NATO surplus are you running? I picked up a bunch of PMC 147gr from the 1980's a few months ago. Haven't shot any of it yet.
 

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Which NATO surplus are you running? I picked up a bunch of PMC 147gr from the 1980's a few months ago. Haven't shot any of it yet.
Currently I'm running through some 70's Indian, it's good for plinking but I have had a few rounds, out of maybe 300 or so I've fired, refuse to fire. Also have a lot of Malaysian and South African on hand, maybe 8-9K rounds of those. Everything else I have is fairly new in comparison and not surplus.
 

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Discussion Starter #10
Maybe the stock military rifle isn't going to be using commercial 308 ammunition which is higher pressure. That means NATO only needs the larger gas port to run in all conditions? Just a theory.

My 17 SBR runs with a 1.45 with NATO surplus M80 in the 12 o'clock position and runs perfect with a Phantom M2 QD in the 10 o'clock position. The 1.45 was just enough for full operation but I might go to a 1.5 just for peace of mind.
This is good data and encouraging. Thanks for sharing. I will only be shooting surplus M80 ball as well through this one (Lake City usually). I'll be sure to come back and share my data after I do some shooting with the 1.50mm.

Cheers!
 

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Civilians (most of us) do not use crap ammo. However, NATO ammo is pretty close to crap unless you get specialized ammo.

I would venture that the reason for the larger gas jet screw is because of the NATO ball ammo that is a much lower pressure gradient that civilization high end ammo. Remember that these are military firearms and they need to operate with a very wide variety of NATO ammo.

Don't worry about what the military gas jet is. Use the gas jet you need for your choice of ammo to function properly. NATO ball ammo for 7.62x51mm is 147gr jacketed. This is a far cry from 168 or 175 grain ammo most of us use as civi's. I personally use 175gr M118 clone ammo although I have over 2500 rounds of M80 ball ammo.

Don't sweat the differences.....use what works for you in your specific platform set up.
 

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Hi everyone,

I'm about to run my SCAR 17S SBR suppressed for the 1st time, and I'm in the middle of swapping out the gas control screw in my factory 13" barrel assembly for a 1.85mm factory screw, which is supposed to be the correct size in a "real" Mk17 CQC. However, after removing the stock gas control screw, I found that it's actually smaller than the 1.85mm one I purchased from Midwestgunworks.com. The stock one looks to be about ~1.60mm.

Why would the real Mk17 CQC have a larger gas jet than the one in the 13" barrel assembly that civilians can buy? From what I had read about the topic previously, I was under the impression that the civilian gas jets were larger because civilians "use crap ammo" and it was to compensate for under-powered target ammo. Or are the civilian gas jets actually smaller to keep from blowing up guns w/ overpowered ammo?

I'm just wondering if I should have left the factory ~1.60mm screw alone and gave it a try first since that would beat the gun up less in either gas setting...

Has anyone else out there experimented with different gas control screws in their suppressed SCAR SBR w/ factory 13" barrel? Would really like to hear what you've learned.

Thanks in advance for any input or advice you might be able to offer!
I run the same setup: SCAR 17, 13" barrel with a Surefire SOCOM RC-2 762 suppressor. I experimented with all jet sizes from 1.25mm up to 1.75mm & settled on 1.45mm. The best source of knowledge & useful information are the videos put out by Jarod over at PARKER MOUNTAIN MACHINE. Take a look at those & you'll be on the road to the proper jet size/suppressor for your gun. A lot of it is trial & error as each rifle is different strangely enough...
 
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