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Thread: why can some 1/9's shoot 75's and 80's and not others?

  1. #11
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    Quote Originally Posted by DMR View Post
    My understanding had always been that the 1-7 was to so they could stablize the longer M-856 tracer rounds. While only a 64 GRN load, the projectile is 1.15" vs. .906" for the M-855. In other words it was less about the weight of the bullet then the length of the bullet and the twist required to stablize it.


    The orig. twist rate was 1/14 and they had issues in cold weather. Then went to the 1/12 twist.

    Later on, they went to the 1/7 to stabilize the tracer rounds. This is all off the top my head so don't hold me to it.



    C4

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    Quote Originally Posted by C4IGrant View Post
    The orig. M-16twist rate was 1/14 and they had issues in cold weather. Then went to the 1/12 twist. For M-193/M-196

    Later on, they went to the M-16A21/7 to stabilize the tracer rounds. For M-855/M856

    This is all off the top my head so don't hold me to it.
    C4
    Of course I'm only a little past hip shooting this one. One of the OTHER experts will be along soon to school me. If you don't first.
    Last edited by DMR; 06-18-09 at 15:42.
    pro-patria.us

  3. #13
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    Grant, The 1 in 14 won't reliably stabilize a 55 grain bullet at 223 velocities. I wonder if the cold aggravated that situation?

    Before I get jumped on, I know "standard" twist for 22-250, 220 Swift, etc. is 1 in 14. It works there because velocity is high enough, or bullets used are shorter than (lighter than) 55 grain.

    I did read that the EX15 (?) had a 14 twist. I'll bet 12 was a lot better. How far back do you go Grant? I remember the AR15's as 1 in 12.

    Mark15

  4. #14
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    Quote Originally Posted by Mark15 View Post
    Grant, The 1 in 14 won't reliably stabilize a 55 grain bullet at 223 velocities. I wonder if the cold aggravated that situation?
    1:14" worked fine for 55gr M193 until the temp dropped and the air became denser. IIRC, the temperature threshold was in the 20's, but I've lost a lot of brain cells since I researched that topic years ago. In "normal" temps, 1:14" had no problems stabilizing the 55gr FMJ's. Of course, M193 is quite a bit hotter than standard .223 ammo in the same weight range.

  5. #15
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    Quote Originally Posted by C4IGrant View Post
    Shoot your gun with 75gr ammo out to around 300yds and see what kind of groups you get (use at least 5rds).



    C4
    i have, sub MOA, around 2.5" about what i get with my 40gr vmax

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    Sometimes you wonder wht they specify what they do, as in the following solicitation:

    https://www.fbo.gov/index?s=opportun...=core&_cview=0

    For the M4A1 carbines specified, a 1:9 twist rate is mandatory:

    "Rifling: 1 turn in 9 (22.8 cm) R.H. twist, 6 grooves and lands (this is an absolute requirement, no other twist rate is acceptable), weight not to exceed 6.5 lbs, mode of operation semi auto and 3 shot burst. "

    Wonder why?
    Last edited by Slater; 06-18-09 at 23:47.

  7. #17
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    Quote Originally Posted by Slater View Post
    Sometimes you wonder wht they specify what they do, as in the following solicitation:

    https://www.fbo.gov/index?s=opportun...=core&_cview=0

    For the M4A1 carbines specified, a 1:9 twist rate is mandatory:

    "Rifling: 1 turn in 9 (22.8 cm) R.H. twist, 6 grooves and lands (this is an absolute requirement, no other twist rate is acceptable), weight not to exceed 6.5 lbs, mode of operation semi auto and 3 shot burst. "

    Wonder why?
    Maybe they will only be using M855 ammunition.

  8. #18
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    Quote Originally Posted by Slater View Post
    Sometimes you wonder wht they specify what they do, as in the following solicitation:

    https://www.fbo.gov/index?s=opportun...=core&_cview=0

    For the M4A1 carbines specified, a 1:9 twist rate is mandatory:

    "Rifling: 1 turn in 9 (22.8 cm) R.H. twist, 6 grooves and lands (this is an absolute requirement, no other twist rate is acceptable), weight not to exceed 6.5 lbs, mode of operation semi auto and 3 shot burst. "

    Wonder why?
    I'll give you guys a little look inside government requirements documents-
    They are, more often than not, written around a specific item that a particular person thinks is the cats a$$. Insertion of seemingly irrelevant or overly specific wording is usually an indication that this is happening. What it does is ensure that only that item can meet the req doc, and therefore is the only itme that can be purchased. This can be a good thing or a very bad thing, depending on who is doing the writing and how much they know.
    Jack Leuba
    Director, Military and Government Sales
    Knight's Armament Company
    jleuba@knightarmco.com

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    Quote Originally Posted by Failure2Stop View Post
    I'll give you guys a little look inside government requirements documents-
    They are, more often than not, written around a specific item that a particular person thinks is the cats a$$. Insertion of seemingly irrelevant or overly specific wording is usually an indication that this is happening. What it does is ensure that only that item can meet the req doc, and therefore is the only itme that can be purchased. This can be a good thing or a very bad thing, depending on who is doing the writing and how much they know.
    Well said.

    This may have something to do with it though:

    The U.S. Department of State, INL Section, U.S. Embassy Bogota, Colombia , has a requirement for the purchase of a six hundred (600) M4A1 type assault rifles, to be donated to the government of Colombia for INL program purposes.
    Might have to deal with more M-193 then M-856. Don't know though, I've never been any further south then Panama.
    pro-patria.us

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    Question A tale of two URs...

    Friend of mine and I bought identicle URs with 1:9 barrels a number of years ago.

    Both URs shot XM193 and Hornady TAP 75gr interchangeably, and grouped well.

    Being my only 1:9 AR, I consider mine to be my "beater"; meaning that if I'm going to be doing lots of drills/rapid fire at moderate distances, that's the gun I use. As such, I have many rounds through it, and it continues to shoot heavy bullets well.

    Friend's UR, a "backup" piece (only shot occasionally) with far fewer rounds, just started "patterning" (uses most of an IPSC target at 100 yards) one cold (mid-30's) day, and has never been able handle heavy bullets since then, under any conditions.

    My beater continues to rack up the rounds and shoot the 75gr well.

    Friend's lightly used/identicle UR started patterning with 75gr, and it was not a gradual thing...like someone threw a switch...one day it was "grouping", the next it was "spraying"....yet it still shoots XM193 with the same accuracy!



    Interesting...

    -Rainman

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