Page 2 of 3 FirstFirst 123 LastLast
Results 11 to 20 of 21

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

  1. #11
    Join Date
    Jun 2006
    Location
    OK
    Posts
    2,385
    Feedback Score
    11 (100%)
    Quote Originally Posted by markm View Post
    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.

    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.
    Mark, you may very well be right. My experience with a cut-down rifle barrel was the opposite; it wouldn't run unless clean, freshly lubed, and with hot ammo.

    Quote Originally Posted by Gutshot John View Post
    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.
    Two thoughts on that - first, I believe that both the pressure and the duration are in play there, so it's not a matter of sufficient pressure alone; and second, I'm not saying the recoil would be more significant than a 11.5"/carbine setup. Maybe it wouldn't even recoil quite as much. What I did intend to convey is that the recoil wouldn't be appreciably less than the equivalent barrel length with a carbine gas system, in which case you would have sacrificed a measure of reliability without gaining an worthwhile trade-off in recoil characteristics.

    Quote Originally Posted by CarlosDJackal View Post
    My 12.7" LWRC is a middy and it works just fine, FWIW.
    Is it a piston-operated system? If so, it's interesting that they chose the middy position for the gas port, but I'm not sure it's apples-apples to compare it with a DI system. In a short-stroke gas piston system it's got a much smaller volume of gas needed to pressurize the piston chamber and force the op-rod rearward.
    Last edited by LOKNLOD; 06-03-10 at 23:30.
    --Josh H.
    Zombies seek out and eat brains. Don't worry; you'll be safe if they attack.

  2. #12
    Join Date
    Nov 2007
    Location
    Houston, TX
    Posts
    431
    Feedback Score
    1 (100%)
    My understanding is that you have to deliver the same volume of gas to the carrier no matter what. You can do that by running high pressure/low volume (short gas/small port) or low pressure/high volume (long gas/big port). The complicated part for me is seeing how dwell time (barrel length after the port) affects things. With a short dwell time you have to deliver the proper amount of gas faster, so a bigger gas port makes sense. A long dwell should need a smaller port. The thing I don't get is how these shorter or longer impulses affect the cycling of the gun. Do you just slow the bolt down with a heavier buffer? Can a 12.5 with a middy gas system beat up the gun like a carbine gas system? It seems like delivering a super-fast burst of lower-pressure gas would be about the same as the slower, but high-pressure impulse of the 16" carbine gas system.

    ....or am I on completely the wrong track...
    I didn't build this post. Somebody else made that happen.

  3. #13
    Join Date
    Jun 2006
    Location
    OK
    Posts
    2,385
    Feedback Score
    11 (100%)
    truthseeker,
    It's not so much the same volume as it is the same amount of force. The gas has to apply enough force to accelerate the reciprocal mass (bolt ass'y, buffer) sufficiently to cycle the action, overcoming inertia, friction, and the recoil spring force.

    The timing issue is also critical in relation to the unlocking of the bolt and extraction of the casing from the chamber. The bolt/extractor have to pull the empty out, and if the system tries to do this before the pressure has subsided, then the casing is locked in the chamber by the outward expansion of the gases inside, and we get some issues. This is why a really heavy buffer and higher extractor tension can help with certain operating systems, like 16" barrels with carbine gas. With the longer barrel and closer gas port, the forces imparted on the BCG are much earlier, relative to the total cycle time, than if the port was farther out with a longer gas system, so it's trying to tear that casing out of the chamber while it's still smashed against the walls by the still expanding gases. We can increase the inertia of the reciprocal mass by using an auto carrier (slightly heavier than a semi version), and heavier buffer, to retard the operation (same force, more mass = less acceleration). The stronger extractor is so we have a better grip on the case rim as we yank it out of the chamber.

    Quote Originally Posted by truthseeker View Post
    It seems like delivering a super-fast burst of lower-pressure gas would be about the same as the slower, but high-pressure impulse of the 16" carbine gas system.
    Assuming "fast" = "early" and "slower" = "later", then sticking with the same terms, the 16" with carbine system is a early burst at high pressure for a long duration. Moving the port to midlength position on the same barrel length would give a later burst, at lower pressure, for a shorter duration. Because the 16/CAR setup is overgassed, this is a positive improvement, moving towards being "optimized". If you keep the 16" barrel, and move the gas port to rifle position, you have a much later burst, at much lower pressure, for a much shorter duration. We're at a point then where the 3 don't necessarily add up to enough "oomph" to properly cycle the action (leading to possible functioning issues). Obviously there are about eleventy-seven combinations to run through.

    In terms of "moving parts" the DI system is pretty simple. But if you think about what a complex dynamic system the AR becomes when you pull the trigger, it really gives you an appreciation for the genius of Stoner and other pioneering gun designers.
    --Josh H.
    Zombies seek out and eat brains. Don't worry; you'll be safe if they attack.

  4. #14
    Join Date
    Nov 2007
    Location
    Houston, TX
    Posts
    431
    Feedback Score
    1 (100%)
    Wow, awesome explanation, thanks!

    Just to make sure I'm following... A 10.5" barrel with carbine gas system should be less of a pain in the butt than a 12.5" barrel with a middy gas system even though the length after the port is about the same. This would be because of the higher starting pressure of the carbine gas versus the middy gas. We can slow down the over-gassed action easier than we can speed up the under-gassed action (theoretically).

    Quote Originally Posted by LOKNLOD View Post
    Obviously there are about eleventy-seven combinations to run through.
    This is what blows my mind. Is there a chart somewhere that tells you what port size, barrel length, buffer, carrier, etc. will work? Is it just trial and error? Voodoo? Power of prayer?


    In any case, the title of this thread needs changing. I don't think this is a stupid question at all.
    I didn't build this post. Somebody else made that happen.

  5. #15
    Join Date
    Mar 2009
    Location
    Deep South Texas
    Posts
    2,521
    Feedback Score
    2 (100%)
    other than trying to save some money for a tube that is on hand for budget project, where is the advantage of a middy gas??

    i'm still wrapping my head around the dwell explanations but isn't a 12.5 in a carbine gas system superior in every way??
    "You cannot invade the mainland United States. There would be a rifle behind each blade of grass."
    Japanese Admiral Yamamoto, 1941




    "A wise man's heart directs him toward the right, but a foolish man's heart directs him toward the left."
    Ecclesiastes 10:2:

  6. #16
    Join Date
    Jun 2006
    Location
    OK
    Posts
    2,385
    Feedback Score
    11 (100%)
    Quote Originally Posted by truthseeker View Post
    Wow, awesome explanation, thanks!

    Just to make sure I'm following... A 10.5" barrel with carbine gas system should be less of a pain in the butt than a 12.5" barrel with a middy gas system even though the length after the port is about the same. This would be because of the higher starting pressure of the carbine gas versus the middy gas. We can slow down the over-gassed action easier than we can speed up the under-gassed action (theoretically).
    Yeah, I think that's kind of the deal.

    This is what blows my mind. Is there a chart somewhere that tells you what port size, barrel length, buffer, carrier, etc. will work? Is it just trial and error? Voodoo? Power of prayer?
    That'd be a heck of a chart. Thankfully most of the "worth the trouble" configurations are out there in one form or the other already.


    I keep waiting for someone who knows more than I do to come in here and point out how horribly wrong I am If anybody's really interested in this I'd encourage them to use what I've said as a starting point and then go research on their on to get their own understanding of the AR system... I'm certainly no authority on the matter.
    --Josh H.
    Zombies seek out and eat brains. Don't worry; you'll be safe if they attack.

  7. #17
    Join Date
    Feb 2008
    Location
    Culpeper, VA
    Posts
    6,313
    Feedback Score
    26 (100%)
    I'm not really looking for a 10.5". 11.5" is my minimum and 12.5" seems to offer most of what I'm looking for as I intend on using the 77gr which seems like kind of a waste out of a 10.5.

    I'm primarily interested in getting better performance with the shootability of the 10.5 and still be relatively reliable.

    I didn't really think that there was going to be significantly more cost/effort to make it so.

    I had thought it was a pretty straightforward proposition. If it's not really feasible than I'll probably go with an 11.5" but that's why this board is a resource.
    It is bad policy to fear the resentment of an enemy. -Ethan Allen

  8. #18
    Join Date
    Oct 2008
    Location
    oregon
    Posts
    7,126
    Feedback Score
    7 (100%)
    well im gonna do one. its about No. 5 on the priority list right now, and i still need to chop the barrel and buy a stripped upper, but shouldnt be too long till i can post some findings.

  9. #19
    Join Date
    Dec 2009
    Location
    Pittsburgh, PA
    Posts
    2,047
    Feedback Score
    17 (100%)
    Quote Originally Posted by Gutshot John View Post
    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.
    Yes, very much so.

    When you build this, I must shoot it.

  10. #20
    Join Date
    Mar 2009
    Location
    Deep South Texas
    Posts
    2,521
    Feedback Score
    2 (100%)
    I can tell you I'm tickled with my 12.5 build. I do however run suppressed mostly...i've tried all kinds of ammo but mostly feed it 75gr reloads. I took a page from Grant's small port research and cut down a carbine gas / 16" dd and running the lmt bcg / bolt per his recommendation. The gun is 100% reliable sans can but bolt does not lock back on an empty mag. With the can, it does. Not 1 failure but I only have a little over 1k rounds down range.

    While this set up is ideal for me, the whole standardized gas port size and location has had my head spinning for custom sbr builds. If the dwell time on a 14.5" middy is considered optimal why not adjust the gas port 'location' for all the other sbr length's??
    "You cannot invade the mainland United States. There would be a rifle behind each blade of grass."
    Japanese Admiral Yamamoto, 1941




    "A wise man's heart directs him toward the right, but a foolish man's heart directs him toward the left."
    Ecclesiastes 10:2:

Page 2 of 3 FirstFirst 123 LastLast

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •