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Can you cut and rethread an AR barrel ?

Discussion in 'Gunsmithing and Repairs' started by Mags, Aug 30, 2009.

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  1. Mags

    Mags Member

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    Trying to go the SBR "budget" route. I have a 16 inch M4 contour barrel I want to cut it down to 10.5 to 11.5. Already aware of the legal implications so no need to talk about the legal side. I just want to know will it affect reliability if I saw off 5.5 inches, smooth, and rethread?
     
  2. rbernie
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    rbernie Member

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    I have no first-hand experience, but I would imagine that if the barrel is chrome-lined, getting a good crown will be much more difficult.
     
  3. Mags

    Mags Member

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    No sir, not chrome lined just a cheap DTI 1 in 9 twist barrel.
     
  4. Seafarer12

    Seafarer12 Member

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    You can cut it down no problem but I imagine you will need to resize your gas port to make up for the lack of the 5 inches of barrel length.
     
  5. Mags

    Mags Member

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    Do you mean the gas port in the barrel? I really don't understand please elaborate.
     
  6. rcmodel

    rcmodel Member in memoriam

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    Very short AR's don't develop enough gas port pressure because the bullet exits the barrel before the pressure has time to go down the gas tube and cycle the action.

    With a standard 16" carbine barrel, the gas port is about 7.5" ahead of the breechface, with another 8.5" of barrel ahead of the gas port. When you cut the barrel off 5", there is so little dwell time with the bullet still in the barrel past the gas port that the action won't cycle.

    This is addressed by drilling out the gas port to give higher pressure over a shorter time frame.

    If you know what you are doing, or don't and get really lucky, you may hit on a combination that works most of the time without beating the gun to death.
    Or not.

    rc
     
    Last edited: Sep 1, 2009
  7. freakshow10mm

    freakshow10mm member

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    Cut, crown, thread 1/2-28tpi. Happens hundreds of times a day.
     
  8. Mags

    Mags Member

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    Thanks RC and Freakshow.
     
  9. Seafarer12

    Seafarer12 Member

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    Like RC Said you will need to open up the gas port to get more gas to cycle the action since it will have less time. I would just find out the size of the port with a sbr and open it up to that.
     
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