Issue installing a CMMG muzzle brake on a PS90 SBR

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Thread: Issue installing a CMMG muzzle brake on a PS90 SBR

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    Issue installing a CMMG muzzle brake on a PS90 SBR

    A friend brought his newly SBR'ed PS90 (barrel cut & threaded by TROS) over for me to assemble for him.

    I attempted to install a NOS (new old stock) CMMG copy of the factory P90 muzzle brake.

    As I tightened the brake it was REAL TIGHT, it was not cross threading. So I pull it off, no damage to the device threads or to the barrel threads.

    I then try to install the 1/2" x 28 TROS muzzle adaptor, it threads on by hand all of the way on.

    At this point I refused to attempt to install the CMMG brake, it was so tight I was using the 15mm wrench to tighten it.

    Anyone know if CMMG used tapered threads on their muzzle brake?
    Last edited by PursuitSS; 10-15-2014 at 11:58 PM.


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    Senior Member vertigosol's Avatar
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    Cmmg has been know to not have good threads on them. I am sure TROS got the threading right on the barrel. As far as tapering your guess would be as good as mine. If I were to run a dedicated flash hider I would just drop the coin on a fh oem part. my 2c
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    If they were tapered ( just like NPT ? ) you should be able to see it / measure it with some Vernier calipers using the top jaws and measuring the minor diameter on the threads in the FH .
    Not easy to explain but could be checked at both ends of the threads. I just couldn't see the design application of a tapered thread on such a component but could see the use of a starting tap and someone in the manufacturing process not setting up a bottoming tap to finish a uniform thread depth .
    Last edited by mjk1; 10-16-2014 at 07:22 AM.
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    I'm sure the TROS threads are cut perfectly as the adaptor screws all the way on by hand with ZERO drag.

    Also, IF the 16.1" PS90 barrels uses the same threads on it as the P90 barrel, it will not screw on the PS90 barrel where the shroud attaches.

    I'll check the threads with my Starrett tonight to see if it tapers, it will start a couple of turns by hand but then gets DAMN TIGHT.

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    Senior Member 9mm_shooter's Avatar
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    Quote Originally Posted by PursuitSS View Post
    I'm sure the TROS threads are cut perfectly as the adaptor screws all the way on by hand with ZERO drag.

    Also, IF the 16.1" PS90 barrels uses the same threads on it as the P90 barrel, it will not screw on the PS90 barrel where the shroud attaches.

    I'll check the threads with my Starrett tonight to see if it tapers, it will start a couple of turns by hand but then gets DAMN TIGHT.
    Both TROS and CMMG say they thread to M12x1 LH, like the original p90 barrel, right? So I agree that someone's threads are out of spec. Since the TROS adapter works, I think it's the CMMG FH. I'd go to a hardware store and find a 12M metric bolt to screw on to the CMMG FH. If it doesn't fit, you can try to send it back to CMMG for rethreading, or you could just use a $10 threading tap to finish cutting the thread and remove the taper.
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    He is talking about my Rifle here....thanks Pursuit-

    ANd thanks Tros for getting the barrel done so quickly...just has the adapter on it at the moment, but would love to get the flash hider put on! Will see if I can find a M12x1LH Tap.

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    They might be a little more than $10.
    If you want ...I will check at work for a good one ? ( PM me if so... I would be more than willing to help out )
    We are a German company heavy " metric " influence with Tooling Standards ect... so we may have 1 I could loan to you ?
    OR Mark at TROS may be able to help also ???

    Even if they are not tapered... the major/minor diameter differences would do it... that is generally considered the " class of fit " which is represented by a " D # " designation on metric taps. It is an H on standard taps.
    Last edited by mjk1; 10-18-2014 at 05:21 AM.
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    Senior Member SONYtec's Avatar
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    Quote Originally Posted by vertigosol View Post
    Cmmg has been know to not have good threads on them.
    This is the first, hearing this.

    I have a CMMG hider on my SBR. The barrel floats in the reciever, not supposed to be tightened down to it.

    It's supposed to bottom out on the barrel. I don't recall it being tapered though, being metric and all.
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    Senior Member vertigosol's Avatar
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    I meant to say cmmg has had a bad run in barrels not being concentric, bores off axis .. thats all.

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    Quote Originally Posted by vertigosol View Post
    I meant to say cmmg has had a bad run in barrels not being concentric, bores off axis .. thats all.
    Haven't heard anything about this in regards to PS90 barrel's.
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