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Thread: Stupid Question - Why wouldn't a 12.5" Middy Work?

  1. #1
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    Question Stupid Question - Why wouldn't a 12.5" Middy Work?

    I understand that current thought is that a 14.5" barrel is the shortest that works with a mid-length gas system but I'm curious why a 12.5" barrel with a 9" gas system (3.5" of dwell time) would be that much different than a 10.5" barrel with a 7" gas system (3.5" of dwell time) if the gas port was properly sized/opened.

    I understand that it would be significantly more reliable with a can on the end but would it work without and still be reasonably reliable?

    PS. Grant this is all your fault for letting me shoot your SBR at the group meet.
    It is bad policy to fear the resentment of an enemy. -Ethan Allen

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    well then here's a little eye-candy for you.





    psych. just a concept photo... this weapon does not yet exist.

    i've sort of been entertaining the idea in my mind off and on. i'm sure you could get one to run. but the 10s are finicky precisely because they have so little dwell.. so you're kind of taking a step backward.

    i'm just trying to figure out how i can justify such an endeavor- what gain could possibly be made? i happen to have a mid barrel that's not being used...

    i'd like to do one just because

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    Quote Originally Posted by bkb0000 View Post

    i'm just trying to figure out how i can justify such an endeavor- what gain could possibly be made? i happen to have a mid barrel that's not being used...

    i'd like to do one just because
    Shooting Grant's 10.5" with a can was a wet dream. I'd like a bit more velocity/range/accuracy that the 12.5" provides with that shoot-ability.
    It is bad policy to fear the resentment of an enemy. -Ethan Allen

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    The closest thing to something like this is an Armstech that uses a gas trap at the end of the barrel. IIRC they mfg a 9.5 or 10.0" SBR version and a 14.5" carbine version.

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    I think it would work, but in order to make it reliable, you'd have to open up the gas port to such a degree that the benefits of the midlength system would be diminished.

    Dwell time is obviously a (the) key issue here, but with the AR DI system, system is the key word, it can't be boiled down to dwell alone.

    Consider what dwell time really is -- yes, it's the time that the bullet is in the barrel past the gas port -- but what's really happening is that it is the time window that the system is allowing the expanding gas to fill the gas tube, and apply a enough force for enough time to operate the action.

    The distance from the gas port to muzzle is probably better represented as the "dwell length" than the "dwell time". The actual time the bullet spends in a fixed "dwell length" will change as a function of barrel length. Think about it - if the bullet is exiting the 10.5" barrel at 2363 FPS or a 16" barrel at 2669 FPS*, that's about a 300 FPS difference. That's a 0.001 sec extra time to travel that same 3.5" of dwell length. That's not much, but if you're operating with a system that the timing is already pushed to the brink of having the enough "dwell time" to function reliably.

    So, for the same dwell length, the longer OAL barrel has less dwell time. Further conspiring against us, is the fact that the longer gas system, needs more volume of gas to expand into the gas tube, and operate the system -- the gas tube just has a larger volume to fill and pressurize before the action can cycle.

    And then another factor working against us, is that the further down the barrel we put the gas port, the lower the port pressure. Which, normally, we think of being a good thing about the mid- or rifle-length systems. But now with our short dwell length, tiny dwell time, and larger volume to fill...we've got less pressure to work with.

    So what do we do? Well, we either increase that "dwell length" (and therefore dwell time) or, since in this exercise we're trying to hold that length fixed, then we enlarge the gas port, to remove a restriction and pressurize that gas system faster to make up for the decreased dwell. The problem now is that you're slamming a bunch more gas back into the action at high velocity and pressure, undoing big part of the whole purpose of going to a longer gas system in the first place - the less violent operating system. And now, you've created a system thats much, much more sensitive to weak loads, temperature variations, a dirty/restricted/poorly lubed system, etc. because it is all so very carefully tuned and timed.

    Summarizing - as you increase the barrel length, and move the gas port out to keep the dwell length the same, you're actually shrinking the dwell time - so to compensate, you have to take drastic measures that really undo the advantages of the longer gas system.

    At least that's my theoretical take on why we're not seeing a lot of 12.5" middys pop up in a world where both 12.5 and midlength seem to be "the new hotness", but you never see them combined.



    *FPS data pulled from some MK262 chrono data from MSTN found on TOS... a 300 FPS difference from 10.5 to 16 was a pretty common figure I found though.
    --Josh H.
    Zombies seek out and eat brains. Don't worry; you'll be safe if they attack.

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    Cool Clear Answers help my feeble mind

    Thanks LocknLoad.
    I now have a better understanding than I thought possible.
    I appreciate your time and consideration in replying with
    what is obviously real life experience and logic.

    That helped me a lot.

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    Locknload- great post.

    I don't see much point for a 12.5" midlength given that the carbine version works so well (so far, anyway). The recoil in my Noveske and DD 12.5" barrels is negligible.

    Use good mags, springs, ammo, etc and you should be good to go.

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    Quote Originally Posted by LOKNLOD View Post
    I think it would work, but in order to make it reliable, you'd have to open up the gas port to such a degree that the benefits of the midlength system would be diminished.
    You'd still have the significantly lower port pressure at 9 inches. I don't think it'd be hard to make run at all. I bet a port of .080-.085 would run fine.

    And then another factor working against us, is that the further down the barrel we put the gas port, the lower the port pressure. Which, normally, we think of being a good thing about the mid- or rifle-length systems. But now with our short dwell length, tiny dwell time, and larger volume to fill...we've got less pressure to work with.
    Man... I don't know... I've seen a Colt rifle barrel cut down to 16" that ran great with a port openned to .090". It's more gas at dramatically reduced port pressure.
    Last edited by markm; 06-03-10 at 10:33.

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    Great post Loknlod. Thanks also to markm.

    I'm not sure that I understand how a the pressure would not be sufficient to cycle the bolt reliably but still be strong enough to make recoil more significant (and the advantages less) than say a 11.5" with a carbine system.
    Last edited by Gutshot John; 06-03-10 at 22:44.
    It is bad policy to fear the resentment of an enemy. -Ethan Allen

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    My 12.7" LWRC is a middy and it works just fine, FWIW.
    We must not believe the Evil One when he tells us that there is nothing we can do in the face of violence, injustice and sin. - Pope Francis I

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