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Thread: Hk MR556 to 416

  1. #31
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    Quote Originally Posted by RegularGuy461969 View Post
    This is very interesting, thanks for posting. My one question or observation if you will; all other reports I've read and heard first hand from those using vented gas blocks on the 10.4" uppers be they cut down 556 barrels or OEM 10.4" barrels state that the vent on the 14.5"/16" gas blocks is too large to fully cycle the bolts unsuppressed. I have an IGF vented gas block that is supposed to be .03 in diameter and my bolt won't cycle unsuppressed. Given, the vent maybe a little wider than reported, I need a set of pin gauges to verfy. But industry not believe it's anywhere near you .075.

    I have two vented gas blocks, one HK OEM (not mounted on any of my uppers yet) and the IGF one. unfortunately, I'm now 16 months into waiting for my SF SOCOM556 SB2 Form 4 to clear (4 months of which was waiting for the Form 3 to clear to my SOT).

    Not trying to argue, just curious of your experience versus others I've read and heard.
    I chose to keep the standard vented gas block and simply open the gas port to compensate. I went single upsize in drill bit until I reached the point that the gun was ejecting (with USGI ejector spring installed) to about the 4 to 4:30. The final bit size used was a #49 at .073" I did not modify the vent on the gas block at all.

    I then attached suppressor an observed eject to about the 2:30 to 1:30 (gun always ejects a little more forward after it warms up which is when it ejects to about the 1:30 to 2:00 position)

    Most important observation regarding ejection is that it was consistent with a healthy throw and ejected in a predictable manner. If brass seems to simply be "falling" out of the side of the gun with an unpredictable throw, this generally means the ejector spring is shot.

    A quick check with the pin gauges shows the standard 416 vented gas block (with sling loops) for a 14.5" barrel to be 1.5mm.

    So, a 10.5" barrel with a .073 (1.93mm port) and a 1.5mm (.0591") vent will work fine for unsuppressed and suppressed.

    Keep in mind that the HK system has two variables that effect function in the gas block and they may or may not work in conjunction with eachother depending.

    If your gun isn't getting enough gas due to small gas port and too short of dwell time, the vent won't mean that much. This is why HK initially had no vent for the 10.4 barrels, thinking that instead of opening the port, they'd just close the vent and call it good. Until people ran them suppressed and the guns started beating themselves to death.

    Experience taught us that the gas port must be opened up to adjust for the lack of dwell time and there must be some form of vent to allow bleed off of pressure from the system once the system actually gets enough gas to run. To accomplish this, HK has a fixed (non agr) block with a smaller gas vent and they've opened up the barrel port (HK now uses a 1.6mm = .063" gas port) to ensure the system has the gas it needs. Screw too much more with one of those without adjusting the other and you lose the "self adjusting" aspect of the gas system and its ability to run both suppressed and unsuppressed.

    With your smaller vent on your gas block, you're now allowing some venting but now don't have enough pressure (small port/short dwell time) to run the system before the vent opens and releases the pressure. Your .03" vent is .76mm which makes just slightly over half the size of your gas port which is running approx. .055" (1.4mm)

    That should tell you something about the efficiency and delicate balance of the system when the port is kept small. That being said, I chose to go with a 416 full vent block and modded the gas port until I got the results I wanted. Same results as HK, different ratios. I prefer a more robust gas system that allows a wider operating range so I went with the larger gas port to match the larger gas vent.

    In the long run this means I'll experience slightly more gas port erosion, but in my years of working .mil guns, I never ONCE coded out an M4/M16 barrel for gas port erosion. Only for physical damage (bullet/frag impact, getting run over by vehicles, etc.), pitting/damage to the chamber or bore, bent barrels, or failed gauge (barrel straightness, bore/muzzle erosion, or headspace).

    The biggest drawback to what I did is that the gas venting out of the front of the gas block has blasted a shiny spot on the rear of my suppressor. Not exactly a show stopper and I really don't care so long as the gun works.

  2. #32
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    Yup....you got me. I was bored, and thought I would just pull one over on you. Dude. Come on.
    Hk MR556 to 416-0721170858a_resized.jpgHk MR556 to 416-0903171802_resized.jpg
    A redneck's famous last words...."Hey everybody, watch this!"

  3. #33
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    Quote Originally Posted by InshallahTech View Post
    I chose to keep the standard vented gas block and simply open the gas port to compensate. I went single upsize in drill bit until I reached the point that the gun was ejecting (with USGI ejector spring installed) to about the 4 to 4:30. The final bit size used was a #49 at .073" I did not modify the vent on the gas block at all.

    With your smaller vent on your gas block, you're now allowing some venting but now don't have enough pressure (small port/short dwell time) to run the system before the vent opens and releases the pressure. Your .03" vent is .76mm which makes just slightly over half the size of your gas port which is running approx. .055" (1.4mm)

    That should tell you something about the efficiency and delicate balance of the system when the port is kept small. That being said, I chose to go with a 416 full vent block and modded the gas port until I got the results I wanted. Same results as HK, different ratios. I prefer a more robust gas system that allows a wider operating range so I went with the larger gas port to match the larger gas vent.
    Thank you for the clarification. You provided some great info that furthers this discussion that has been the subject of several threads over the years.

    I have one question: Did you have any issue with chatter when boring the gas port? I've talk to someone about doing this, he did not want to do it due to his belief there would be too much chatter; he didn't want to take the chance of screwing up my 416 barrel.

    Thanks Again for the info.

    RG
    "Just because you're paranoid doesn’t mean they're not after you!"

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  5. #34
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    Quote Originally Posted by Brad4065 View Post
    Yup....you got me. I was bored, and thought I would just pull one over on you. Dude. Come on.
    Click image for larger version. 

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    I'm curious as to what ammo you're shooting ? Thx !
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  6. #35
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    Quote Originally Posted by Crashmc View Post
    I'm curious as to what ammo you're shooting ? Thx !


    Hk MR556 to 416-0924170912_resized.jpg

    And the only reason I know I've gone through approximately 3K rounds also.

  7. #36
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    Quote Originally Posted by RegularGuy461969 View Post
    Thank you for the clarification. You provided some great info that furthers this discussion that has been the subject of several threads over the years.

    I have one question: Did you have any issue with chatter when boring the gas port? I've talk to someone about doing this, he did not want to do it due to his belief there would be too much chatter; he didn't want to take the chance of screwing up my 416 barrel.

    Thanks Again for the info.

    RG
    I literally used a tap handle and high speed drill bits to open the port up one size at a time. When done in this manner, it is self centering and there is no need to go through the machine shop setup. The amount removed with each upsize is on average between 35 and 40 thou (give or take) per upsize which really isn't much at all which also helps prevent heat buildup, chatter, centers the bit, and requires very little force which means less wear on the bit, less chance of breaking the bit, very little if no flash inside the bore once you break through etc. Just go slow.

    On the other hand, I experienced serious bit chatter when I first started turning down the barrel profile. Although I don't know the exact material type, I suspect that there's a fairly high nickel content in HK barrels and they seem to work harden. So in my experience, lathe work needs to be done relatively quickly and steadily because it seems to harden as you're machining and cause bit chatter if you go too slow. Carbide did work better and made cleaner cuts.

  8. #37
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    His gun is beating itself to death.
    I really have not shot mine enough to show a lot of wear but there is wear on the front edge of the deflector and 0 wear on the ejection port. This is a cut down to 10.4" MR556 barrel with a non vented 416 gas block which is all there was at the time.
    What is your opinion on whether or not I need a vented gas block. I do not have a suppressor and don't want one.
    What is the gas port size on a MR556 barrel. I showed my upper to a class 3 dealer and he said it was wearing in nicely. Thanks.
    Last edited by Tuvia; 09-25-2017 at 07:00 PM.

  9. #38
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    Quote Originally Posted by Tuvia View Post
    I really have not shot mine enough to show a lot of wear but there is wear on the front edge of the deflector and 0 wear on the ejection port. This is a cut down to 10.4" MR556 barrel with a non vented 416 gas block which is all there was at the time.
    What is your opinion on whether or not I need a vented gas block. I do not have a suppressor and don't want one.
    What is the gas port size on a MR556 barrel. I showed my upper to a class 3 dealer and he said it was wearing in nicely. Thanks.
    The ejection pattern is supposed to be between 3 and 4:30. If yours is close to that, it's fine. I would recommend using the heaviest buffer that will still allow reliable function. You could also go to the Vltor A5 system if you're really concerned.

  10. #39
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    Quote Originally Posted by InshallahTech View Post
    On the other hand, I experienced serious bit chatter when I first started turning down the barrel profile. Although I don't know the exact material type, I suspect that there's a fairly high nickel content in HK barrels and they seem to work harden. So in my experience, lathe work needs to be done relatively quickly and steadily because it seems to harden as you're machining and cause bit chatter if you go too slow. Carbide did work better and made cleaner cuts.
    Haul the mail at 500-800SFM with high positive coated carbide and you will get great results.

  11. #40
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    Quote Originally Posted by Tuvia View Post
    I really have not shot mine enough to show a lot of wear but there is wear on the front edge of the deflector and 0 wear on the ejection port. This is a cut down to 10.4" MR556 barrel with a non vented 416 gas block which is all there was at the time.
    What is your opinion on whether or not I need a vented gas block. I do not have a suppressor and don't want one.
    What is the gas port size on a MR556 barrel. I showed my upper to a class 3 dealer and he said it was wearing in nicely. Thanks.
    If you don't plan on running a suppressor, I'd say "No", but do keep an eye out for heavy wear. Check the nylon insert at the rear of the buffer for impact mushrooming, watch your ejection, change action spring every 5k to 7.5k rounds with genuine HK replacement just to make sure. I do also suggest changing out the ejector spring with a milspec DI ejector spring if you actually plan on watching ejection as part of your PMCS diagnosis. The HK spring is quite a bit lighter then a USGI milspec ejector spring and does not give a proper diagnosis of bolt speed, etc.

    If you do change your ejector spring to a milspec version and see your ejection heading to the 1:00 to 1:30, you might want to think about first changing your action spring and if that doesn't do it, perhaps consider the 10.5 vented block, but also realize that for that to properly work your port will need to be opened up to 1.6mm.

    Also understand that ejection pattern as a diagnosis tool is limited and works best when the rifle is working close to milspec performance metrics (bolt speed, USGI parts, etc.) If you continue to use the HK ejector spring, understand that ejection pattern diagnosis no longer really applies because it throws brass slower and therefore makes your gun seem overgassed even if it isn't.

    Your gas port (unmodified) is .055 (1.4mm) Don't worry about port erosion.

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