Barrel Length Question

Discussion in 'AR-15 Discussion' started by Schmeekey, Aug 3, 2011.

  1. Schmeekey

    Schmeekey New Member

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    If a carbine gas system can accommodate a 10.5 inch barrel why is it that the shortest a mid-length can accommodate is a 14.5?
     
  2. JonM

    JonM Moderator

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    because 14.5 is about midway tween 10 and 20 hence the label mid length. carbine length wasnt meant for a 16inch barrel its meant for an actual M4 military barrel which is 14.5 inches the extra length 4.5 at the end is for a bayonet.

    16 inch barrels would perform better with a midlength giving a smoother gas impulse and longer sight radius.
     

  3. Schmeekey

    Schmeekey New Member

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    Thanks for the answer, but that wasn't really what I was asking. I was wondering why a carbine length can semi reliably run a barrel that ends one inch away from the gas block. By that logic, you would think that a middy could do the same. But as I have been told, this is not possible and that the shortest you can have is 14.5. I am simply wondering why you can't use a 12.5" barrel for example. I hope this clarified my question.
     
  4. Gatoragn

    Gatoragn Active Member

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    I don't know if this is relevant to your question or not.

    According to the owner of my favorite LGS, the shortest legal rifle barrel for civilian use is 16 inches. 14.5 inch barrels accomplish this by having a muzzle device (flash hider, brake) pinned or welded to meet the 16 inch minimum.

    To be a legal rifle, a 12.5 inch barrel would need a 3.5+ inch muzzle device.
     
  5. TekGreg

    TekGreg Lifetime Supporting Member Lifetime Supporter

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    Schmeekey, even though the short barrel terminates one inch from the gas block, the distance the gas has to travel back in the gas system is much less in the short barrel system. As you lengthen the barrel AND the gas tube, the bullet must remain in the barrel longer past the gas block to pressurize the longer tube to reliably cycle the gas system. That extra couple of inches allows time for the gas to cycle back down the longer tube and do all that it needs to do! :cool: I know that manufacturers test different lengths and sometimes cannot get certain lengths to function because of recoil/gas pulse problems based on length, tube size, gas block, etc. Gas systems are a constant compromise and balancing act, and that is probably why you are not seeing the "reasonableness" of this build. You may be able to change the tube and/or the block and get a better result from a custom build.

    I hope that helped!
     
  6. JonM

    JonM Moderator

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    the closer the port is to the chamber the higher the gas pressure thats why carbine ports are smaller than rifle 20" ports. takes less gas to operate the carbine tubes due to the increased pressure
     
  7. Schmeekey

    Schmeekey New Member

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    Thanks! this totally answered my question. I was just curious to why no one had made a shorty mid-length. Thanks for such a detailed answer.
     
  8. TekGreg

    TekGreg Lifetime Supporting Member Lifetime Supporter

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    An even better response

    Schmeekey, I did some more research and found out some more information for you.

    As the bullet is fired, the brass casing is under extremely high pressure and is locked into the chamber. As you know, when the bullet passes the gas port, escaping gasses go up into the gas tube and back toward the bolt and bolt carrier, forcing them to unlock and move back to cycle the action. However, if the bullet is still in the barrel when the bolt tries to unlock because there is a lot of barrel length it still has to travel before it exits, then the case still has too much pressure to release the chamber and the gun may not cycle. The opposite is also a problem when the barrel is very short. If the bullet exits the barrel too soon after passing the gas port, then you have too little dwell time and insufficient gas going down the tube back into the bolt and bolt carrier (since more more gasses end up escaping out of the end of the barrel). This also causes malfunctions. Again, this can be adjusted in a number of ways, size of gas port, size of gas tube, position of gas block, length of tube, etc.

    This is the best answer I could get for you at the moment. Stay tuned for further updates if I come up with something even more detailed - I may call Colt and talk to their gas specialist. :eek: :cool:
     
  9. Schmeekey

    Schmeekey New Member

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    Thank you again for putting in time to answer my question.
     
  10. Quentin

    Quentin New Member

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    Great answers, TekGreg!

    Just repeating some of the points, you probably can get a midlength gas system to work with a barrel shorter than 14.5" but it would take a lot of tuning to make it reliable with various ammo, increasing the gas port and playing with the buffer weight and spring, maybe even a light weight bolt carrier. Even in a 14.5" sometimes it's hard to get Wolf and other light powered ammo to be reliable with midlength gas.

    It's just not worth it so you see the carbine length gas in shorter barrels.
     
  11. TekGreg

    TekGreg Lifetime Supporting Member Lifetime Supporter

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    Quentin, absolutely correct, from what I can tell. The more tuning you do, the more specialized the platform becomes and the more finicky it will be about accepting types of ammunition and bullet weights. Manufacturers have to make a gas system that will devour every known weight/powder combo on the market and therefore are extremely limited by what can be put on a production gun.
     
  12. Quentin

    Quentin New Member

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    Very true, and that's why so many manufacturers have boring guns, they've got to work with almost all ammo a customer is likely to use, otherwise customer support will be a nightmare. When someone takes it upon themselves to assemble their own AR, he has to get up to speed quick on many issues or end up with that "finicky gun" no one wants. Tuning is great if you know exactly what's going on but for the average person it adds headaches.