Page 3 of 5 FirstFirst 12345 LastLast
Results 21 to 30 of 48

Thread: barrel length, gas system, dwell time, accuracy

  1. #21
    Join Date
    May 2006
    Location
    VA/OH
    Posts
    29,630
    Feedback Score
    33 (100%)
    Barrel length has nothing to do with accuracy.


    C4

  2. #22
    Join Date
    Jun 2006
    Location
    NoVA
    Posts
    10,780
    Feedback Score
    17 (100%)
    Quote Originally Posted by Son of Vlad Tepes View Post
    I don't mean to hijack the thread, but one quick question:

    Are there any advantages of using a midlength gas system in an 18" barrel compared to using the new intermediate gas system?
    The intermediate was made for a 18" barrel, if it's available in the barrel you want to get I'd highly recommend getting it.
    The midlength will shoot well with an 18" barrel but recoil well be a tad more harsh and the pressure a little more than ideal, midlength was really meant for 16" barrels and if you look at KACs version it's even longer than the other ArmaLite system used by everyone else and it shoots softer better still.
    Chief Armorer for Elite Shooting Sports in Manassas VA
    Chief Armorer for Corp Arms (FFL 07-08/SOT 02)

  3. #23
    Join Date
    Nov 2007
    Location
    North AZ
    Posts
    630
    Feedback Score
    0
    Thank you very for the information guys.

    The intermediate was made for a 18" barrel, if it's available in the barrel you want to get I'd highly recommend getting it.
    I'm actually trying to figure out if I should have my 18" (Noveske mid gas length SPR upper) cut down to 16", or to just fogetaboudit and keep it as is.
    Formerly known as "Son of Vlad Tepes"

  4. #24
    Join Date
    Feb 2010
    Location
    Squirrel!
    Posts
    2,156
    Feedback Score
    3 (100%)
    Quote Originally Posted by Sundo View Post
    The Mk12 SPRs have an 18" barrel with an intermediate length gas system (between mid length and rifle length), so the dwell time on the SPR 18" barrel with intermediate gas system should be similar to that of a 20" barrel with rifle length gas system or 16" barrel with midlength gas system.
    No, military SPRs use rifle-length gas systems.

  5. #25
    Join Date
    Apr 2010
    Posts
    20
    Feedback Score
    0
    Quote Originally Posted by Molon View Post
    For starters, your numbers are WAY off....
    Sorry, I was listing milliseconds, not seconds. My mistake.

    The table I've been referring to is here:



    For the "proper length gas system" (14.5 barrel, carbine length gas system; 16 inch barrel, midlength gas system; 18 inch barrel, intermediate gas system; 20 inch barrel, rifle gas system) the dwell time ranges from 0.181-0.206 ms. The 16 in barrel with carbine gas system or the 18 inch barrel with midlength gas system have the two longest dwell times: 0.248 and 0.234 ms, respectively. An 18" with intermediate gas system has a dwell time of 0.181 ms.

    The difference between the dwell times of a 16” barrel and an 18” barrel that both have a mid-length gas system will be approximately 0.06 milliseconds; that’s 0.00006 seconds.
    While that difference of 0.053 ms is small compared to our normal sense of time, that is a 30% increase in dwell time compared to the 18" barrel with intermediate length gas system. I'm assuming the operation of the gas system introduces disturbances into the barrel motion that are not there before the start of dwell time. Thus, longer dwell times should inflict more disturbance on the barrel while the bullet is between the gas port and muzzle. Is this not reasonable?

    I'm interested in a SPR/DMR type of rifle, so that last bit of accuracy is important to me, if it is measurable.

    ...
    A. I’ve never seen a single study showing a causal relationship between an increase in the amount of time that it takes the bullet to travel from the gas port to the muzzle and a decrease in accuracy in an AR-15.
    I haven't seen formal studies, either. I have seen magazine article reviews of 14.5" carbine length gas system rifles having better accuracy than very similar 16" carbine length gas system rifles. This data, by itself, isn't conclusive. So, I'm hoping somebody has tried a side-by-side test.

    B. “All else” is not equal.
    .....
    I'm considering a rifle that is otherwise identical, except one version has a 16" barrel and another version has an 18" barrel. The rest of the upper receiver (including gas system length) is identical.

  6. #26
    Join Date
    Nov 2008
    Location
    NM
    Posts
    3,988
    Feedback Score
    10 (100%)
    Quote Originally Posted by C4IGrant View Post
    Barrel length has nothing to do with accuracy.
    Correct.

    Dwell time has nothing to do with accuracy for practical terms, disregard that. Dwell time is important in being able to consistently cycle the bolt, but doesn't affect internal or external ballistics in any noticeable way.

    As much as you'd like to believe that those minutae might matter: they won't. You're not going to get below 3/4MOA 10-shot groups with an semi AR-15, and that's with a well developed load for it.

    If you're comparing a true apples to apples 16" and 18", and weight isn't a factor just get the 18" for the added barrel length and be done. From all your other information, those comparisons aren't really oranges to oranges. To really even get much use out of a precision rig like this I'm assuming you could do with the extra muzzle velocity, so 18" might fare better.
    Last edited by TehLlama; 05-30-10 at 19:56.
    عندما تصبح الأسلحة محظورة, قد يملكون حظرون عندهم فقط
    کله چی سلاح منع شوی دی، یوازي غلوونکۍ یی به درلود
    Semper Fi
    "Being able to do the basics, on demand, takes practice. " - Sinister

  7. #27
    Join Date
    Apr 2008
    Location
    WA
    Posts
    28
    Feedback Score
    0
    Quote Originally Posted by TehLlama View Post
    Dwell time has nothing to do with accuracy for practical terms, disregard that. Dwell time is important in being able to consistently cycle the bolt, but doesn't affect internal or external ballistics in any noticeable way.
    So, what is the relationship between dwell time and reliable operation?

  8. #28
    Join Date
    Jun 2006
    Location
    Colorado Springs
    Posts
    1,857
    Feedback Score
    0
    Quote Originally Posted by pinepig2 View Post
    So, what is the relationship between dwell time and reliable operation?
    You need enough dwell time to charge the gas system to ensure reliable cycling. Too much dwell (i.e., too much gas) stresses the system and is hard on parts.

    Dwell and gas port diameter have to be optimized to get the correct volume of gas. If you have short dwell you need to open up the port some. If you have excessive dwell, then you'd need a smaller diameter port to regulate the amount of gas being sent back to the carrier. It all goes hand in hand.

  9. #29
    Join Date
    Sep 2008
    Posts
    68
    Feedback Score
    1 (100%)
    Although it sounds as though gas port location/dwell time have no measurable effect on accuracy in the AR15's direct gas action, the theoretical question isn't so far-fetched. According to noted Garand-smith Clint Fowler, gas timing can have a significant effect on the accuracy in the Garand:

    http://clintfowlerrifles.com/2008/07...arand/#more-13
    I became aware of it after building a rear lugged M-1 with a Krieger barrel for a customer in Washington State. On the test bench it shot 2¼ inches. For a Krieger barrel this was not acceptable so I set the gun aside with a feeling of frustration. I knew the gun had been built with care. I knew the barrel was first class. It had to be something else. A week later the hair-top computer spun out the answer. It was timing. The bullet was still caught in the rifling when the op-rod started to move rearward. It set up barrel vibrations, which affected accuracy. This explained all those guns with good components, which did not shoot well. They could all have been made to shoot good if a way had been found to delay the op-rod function until the bullet just cleared the muzzle...

    The solution was an adjustable gas plug. A plug with a threaded bore with two setscrews mounted inside. By adjusting them outward the volume was increased. More volume, more time needed to fill it. Time measured in parts of a millisecond. To say this solution worked is an understatement. It was the missing link, the last rear hurdle...
    Please note that this is Mr. Fowler's claim, not mine.

  10. #30
    Join Date
    Nov 2008
    Posts
    1,343
    Feedback Score
    0
    Quote Originally Posted by beckman View Post
    Although it sounds as though gas port location/dwell time have no measurable effect on accuracy in the AR15's direct gas action, the theoretical question isn't so far-fetched. According to noted Garand-smith Clint Fowler, gas timing can have a significant effect on the accuracy in the Garand:

    http://clintfowlerrifles.com/2008/07...arand/#more-13

    Please note that this is Mr. Fowler's claim, not mine.
    Please note that the AR-15s being discussed do not have an operating rod.


    .....
    All that is necessary for trolls to flourish, is for good men to do nothing.

Page 3 of 5 FirstFirst 12345 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
  •