14-16 inch AGB
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Thread: 14-16 inch AGB

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    Default 14-16 inch AGB

    Maybe a stupid question. Can you use a 14-16 AGB on a 10.4 barrel?

    Any issues? Modifications needed?
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    Quote Originally Posted by Guido02 View Post
    Maybe a stupid question. Can you use a 14-16 AGB on a 10.4 barrel?

    Any issues? Modifications needed?
    You have increase the bore of the barrel gas port/vent. I think its out to 1.9mm but I’ve yet to have it done. That is the diameter I believe a regular 14.5/16” gas block needs on a 10.4” barrel. I assume that’s what you would use for the AGB as well. YMMV.
    Last edited by RegularGuy461969; 08-09-2019 at 09:56 AM.
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    It needs to be opened to at least 1.6mm

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    Quote Originally Posted by tlrgsxr View Post
    It needs to be opened to at least 1.6mm

    That's for a vented 10.5 GB.

    For a vented 14.5/16 GB, it is significantly larger if one wishes to use the factory buffer and action spring.

    RG has it right.

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    The AGB (adjustable) has selector port sizes of 1.6mm or 1.4mm, thus the barrel port needs to be opened to at least 1.6mm to work as intended. If it isn’t, then the barrel gas port will be more restrictive than the selector. If opened larger, the selector will be the restriction. What does a 1.9mm port do for you if it’s immediately restricted to 1.6mm or 1.4mm?

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    The 1.6mm and 1.4mm specifications you list are for the 10.4 AGB. It also has a significantly smaller gas vent than a 14.5 GB.

    1.6mm unsuppressed, 1.4mm (restricted flow) for suppressed.

    A 14.5 GB has a significantly larger vent due to the increased dwell time.

    Using a 10.4 AGB on a 14.5/16 barrel will result in massive overpressure regardless of the setting.

    It would logically track that an AGB for a 14.5 would have similarly sized vents and gas ports based on the non-AGB specifications to account for dwell time.

    That being said, using a 14.5 AGB on a 10.4 would require similar port modification to account for the excess gas bleed off from the larger vent.

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    I’m referring to the Norwegian AGB intended for longer barrels (not 10.4), they have 1.6/1.4mm selector and 1mm vent. They can be used on a 10.4” barrel provided the barrel gas port is opened to at least 1.6mm like I mentioned. In fact, the vented gas block for the 10.4” barrel uses a 1.6mm gas port and a 1mm vent.

    I measured the port sizes I mentioned with pin gauges. Can you post the selector and vent sizes for the 2 AGBs you’re referring to? Thanks.

    Edited to add: what factory HK barrel has a 1.9mm gas port?
    Last edited by tlrgsxr; 08-10-2019 at 11:39 PM.

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    Sigh. You are mistaken that the AGB's you talk of are for 14.5 guns. They are not.

    The vented non-AGB for the 10.4 has a 1.6mm barrel gas port with a 1mm overall vent. Meaning that it retains positive pressure due to the short dwell time, yet does allow bleed off compared to the original non vented 10.4 GBs.

    The available 10.4" AGB's have the 1.6mm/1.4mm vent settings with a 1mm overall exit vent, and require a 1.6mm gas port in the barrel. Meaning that regardless of the setting, the system retains positive pressure due to the short dwell time of the 10.4 barrel, yet slightly restricts gas flow on the 1.4mm setting to account for suppressor use.

    Now stay with me here....

    The non AGB 14.5/16 GB's have a 1.4mm overall vent and require a 1.4mm gas port on the barrel. Meaning that the system reaches a neutral pressure once the vent opens due to the extended dwell time compared to the 10.4 GBs.

    To use a non-modified vented 14.5 GB on a 10.4 gun, the barrel gas port MUST be opened up to 1.9mm when using a factory buffer/spring combo otherwise the gun will be under gassed.

    Thereby it logically tracks that if an AGB made for a 14.5/16 gun with appropriate dimensions to use the 1.4mm gas port and account for the extended dwell time and has a similarly larger (1.4mm) overall vent, would still require the 1.9mm gas port on the barrel if used on a 10.4" gun.

    An AGB for a 14.5 gun would have the large setting of 1.4mm (to match the barrel port) and a smaller setting (most likely around 1.2mm) for running suppressed. Not a 1.6mm "normal" setting and the exact same adjustment port size as gas port size, and retain the 1mm overall vent at the same time.

    The mere fact that the AGB's we've seen thusfar have a 1mm overall vent is alone enough information to know that they are not appropriate for a 14.5/16 inch gun.

    Especially considering that the Norgs simply changed gas blocks on the 14.5 guns, not barrels, simply for the sake of having a more durable AGB, not for function issues.

    To assume that the AGB's you find are suitable for the 14.5/16 barrels is a little silly given the dimensions and dwell time involved. Especially considering that nowhere has HK mentioned opening the gas port on a 14.5/16 barrel to 1.6mm. This has ONLY been the case for tuning a 10.4 barrel to run with a newer vented GB.

    We can break it down to a "pressure retention" system.

    14.5/16 + standard vented GB = 1.4mm gas port - 1.4mm gas vent = neutral pressure when vent opens.

    10.4(original) + standard non-vented GB = 1.4mm gas port - 0mm gas vent = +1.4mm of positive pressure retained(the originally severely overgassed system that lead to excessive bolt speeds and broken discos).

    10.4(new) + vented GB = 1.6mm gas port - 1mm gas vent = .6mm positive pressure when vent opens.

    10.4 + AGB on unsuppressed setting = 1.6mm gas port - 1mm gas vent = .6mm positive pressure when vent opens.

    10.4 + AGB on suppressed setting = 1.4mm restricted gas port - 1mm gas vent = .4mm positive pressure when vent opens.

    10.4 (original) + 14.5 standard vented GB = 1.4mm gas port - 1.4mm gas vent = zero positive pressure with minimal dwell time and excess bleed off after vent opens.

    10.4(new) + 14.5 standard vented GB = 1.6mm gas port 1.4mm vent = .2mm positive pressure with minimal dwell time and excess bleed off after vent opens.

    10.4 (modified) + 14.5 standard vented GB = 1.9mm gas port - 1.4 vent = .5mm positive pressure on vent open. (the minimum required to run well and reliably lock back on last round of the magazine)

    The 10.4(modified) with 14.5 vented GB and a 1.9mm gas port run quite well both suppressed and unsuppressed without being over gassed. For those who do not wish to run suppressed, I would suggest a 2mm barrel gas port to get the .6mm retained positive pressure in the system.

    Were there to be a dedicated AGB for a 14.5/16 barrel, it would have to have a restricted setting >1.4mm and retain the 1.4mm vent thereby having a negative pressure on vent open to account for the extended pressure spike caused by the use of a suppressor, OR if HK did call for the gas port to be opened to 1.6mm on a 14.5/16 barrel, the overall exit vent on the GB would have to be 1.6mm as well.

    Were you to place a 10.4 AGB on a 14.5 barrel, you would lose the neutral pressure vent open system and have 4.1 inches of dwell time at .4mm positive pressure regardless of the setting you placed the selector at. Meaning it would be a wash regarding exit vent and the system would now retain positive pressure beyond what it was designed to handle.

    And if you were to open the barrel gas port to 1.6mm, now the system has an extra .6mm positive pressure on the unsuppressed setting thereby overgassing the hell out of it even more.

    Logically, your line of thinking simply does not track with the proven operating parameters of the 416 gas system.
    Last edited by InshallahTech; 08-11-2019 at 09:32 PM.

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    That’s all great in theory, and you have a lot of them, but there’s no facts there. Maybe you could start by posting the valve sizes for the long barrel AGBs? Or even which HK barrel has a 1.9mm port? Or for that matter, any facts at all??

    I know, you’re always right regardless. You suggested a 1.9mm gas port for a long barrel AGB without even knowing the port sizes, WTF?!?

    Carry on, it’s all yours.

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