Jamming problem on new Scar 17 / 13" barrel

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  • 3 Post By STL
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Thread: Jamming problem on new Scar 17 / 13" barrel

  1. #1
    Junior Member
    Join Date
    Mar 2017

    Jamming problem on new Scar 17 / 13" barrel

    Hey all,

    Hate that my first post is not a positive one but I just got my 17 set up with a 13" barrel (waited a lonnnng time for the stamp) and it's having problems. Here's the lowdown (I'll follow forum rule 21 on posting about problems):

    Brand new Scar 17
    Ammo used was PMC .308 147gr, Federal XM80C 7.62x51 149gr and Winchester Western 7.62x51 147gr
    Brand new factory magazine
    Spring, TX
    Rifle bought off gunbroker (new in box)
    ~70 rounds through rifle when problem occurred
    Photo is below
    Will call FN tomorrow to report issue
    Problem has now happened twice, first time Sat 3/11, second time today 3/13

    So I've taken the rifle out three times. First time was about 3 weeks ago, I ran 40 rounds of PMC through it with no issues (10 with a YHM Phantom ULT can, 30 no can) and it ran perfect, I was very happy. Second time out (Sat 3/11) I ran about 30 rounds of PMC through it and then a spent casing got jammed in the ejection port. I cleared it, loaded up five more PMC and on the second round the exact same thing happened. I cleared it again and switched to the Federal ammo and on the third round the exact same thing happened so I called it a day. I cleaned the entire weapon yesterday including the piston and gas block and took it out a few hours ago. I ran 20 rounds of Federal through it (10 with the can, 10 no can) and it ran fine. I switched to the Winchester ammo and on the third round the exact same thing happened, the picture is of the jam today (I don't have pics from Saturday but it was the exact same problem).

    It's a new 13" factory barrel which I installed myself along with a Kinetic rail. Just in case anyone asks yes I had the gas regulator set straight up when I was shooting without the can which is when the jams happened. Here's a pic, any help would be greatly appreciated, if any more info or pics are needed I'd be happy to share:


  2. #2
    STL is offline
    Senior Member 25,000 Post Club Member STL's Avatar
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    3rd rock from the Sun
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    Your weapon is short-stroking on occasion with that ammunition and it is an ammunition issue. If you'd prefer to replace the gas control jet to the next larger orifice in order to ensure positive ejection due to variables in pressure with the PMC ammunition, you can find them at Midwest Gun Works.

  3. #3
    Junior Member
    Join Date
    Mar 2017
    Ok thanks STL, my concern is that this same exact jam happened with three different types of ammo, not just the PMC. And from what I understand what is supposed to be one of the excellent attributes of the scar platform is that it isn't ammo "sensitive". I've read a little bit about the gas control jets, I'll dig into it deeper and see what happens.

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  5. #4
    Join Date
    Dec 2014
    St. Louis, MO
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    It's more than likely undergassed because you installed a shorter barrel. You need different jets.

    PMM SCAR 16/17 Stainless Steel Gas jets
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  6. #5
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    Mar 2017
    Thanks ImageX, I'm going to do some research on this and try to determine the optimum jet size that will work for the largest range of ammo, if I have to I'll work it from a "test and tune" angle. If worst comes to worst I'll tune it for one brand / weight of ammo and buy a whole bunch of it (won't be the first time). I appreciate the link to the PMM site, I'll hit them up as well maybe they have some recommendations. My logic when doing this build was to buy the factory stock 13" barrel as I thought (considering their resources) FN would have figured out the correct jet size for the short barrel and I wouldn't have to deal with any issues. The more builds I do the more it seems to be this is never the case but it's just physics so I'm hoping I can figure it out. Right now I'm just kind of bummed because I was hoping this would be a slam dunk gun and would run without problems with any ammo you throw at it. Ah well, as the saying goes, if it was easy....

  7. #6
    FN Supporter
    Jarod Legsdin's Avatar
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    Nov 2013
    Strafford NH
    1675 times
    STL, is correct, in the past 2 years I have Cut and Re threaded over 500 SCAR barrels(16&17) Typically I send the 17 13" PMM CQC barrels out with a 1.8mm Gas jet installed(as well as a 2nd Gas jet if needed 1.6mm & 2.0) this is a tad bit Over gassed for the 13" however in my experience the gun will run with a variety of ammo.

    With that said all my in house testing was initially conducted using the GEMTECH HD series of Suppressors (Dagger& ONE) as well as Several Surefire offerings (SOCOM 762, 300SPS & Genesis) the 13" guns seem to typically like the 1.6mm to 1.8mm Gas jets.

    Naturally the sizing of the jet will change based on the Suppressor manufacturer's design this is attributed to 3 primary factors, Baffle design and Internal volume.
    Both change the back pressure your Gun will see as well as the final and most critical factor the ammo.

    Most of my silencerCo customers run a 1.2mm/1.3mm Gas jet which is very small.

    I personally do not mind having my guns overgased as I know they Will run regardless of what I feed them and the SCAR can take it.

    In the process of "tuning" your gun you may want to find a more common ground for the gas, Generally speaking I test the guns as follows:

    on a full mag with 1 round chambered,Will it fire and cycle the next round with full mag spring tension? Yes-Move on.

    Fire the gun with 1 round chambered on an empty mag,does the bolt lock back" Yes-Move on.

    At that point you need to evaluate the overgased volume bleeding out through the ejection port, which on the SCAR should be little to none.
    If there is a ton of gas coming out of the ejection port it is time to step down a size in the jet and repeat the test process verify both afor mentioned conditions.

    Bare in mind Temperature variables will play a role in this process as well. In the cold here in NH at 10*-25* the 11"PMM SCAR 17 has a tendency of sending more Gas out of the ejection port with the 300SPS, however this is not the case as it warms up.

    Finally Clean the gas Selector Valve.
    The amount of gas on the SCAR is directly related to this and the fouling/carbon build up that may occur on the selector/valve it will effect the weapon. The image below is a Barrel that was sent to me with a "cycling" issue. the smaller port was almost plugged with carbon.

    hope that helps~Jrod.

    gas valve.jpg
    Last edited by Jarod Legsdin; 03-14-2017 at 12:26 AM. Reason: image
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  8. #7
    Junior Member
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    Mar 2017
    Excellent info Jarod, thanks! I will be ordering some new PMM jets shortly. I talked to FN customer service a little while ago and I was told that the jet size in the factory 13" barrel could be 1 of 3 sizes: 1.55, 1.60 or 1.65 mm which I thought was interesting. I totally agree with your analysis on the suppressor / jet sizing interaction which I'll have to work through after I get the rifle functioning properly without a can. Your testing protocol, overgas volume evaluation and selector valve cleaning recommendation is also well received. And thanks STL and ImageX as well, I'll post back with results when I get some testing done with larger jet sizes.

  9. #8
    Junior Member Tek74's Avatar
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    Feb 2017
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    Good info here for future reference. Glad I saw this.
    Scar 17S accessorized with Elcan 1.5-6X, Geissele Trigger, PMM Magpul Miad, KDG QD Mounts, KDG Charging Handle, and Grip Pod V2

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