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Yet another Colt AR question...

Discussion in 'Rifle Country' started by MrTwigg, Jul 30, 2009.

  1. MrTwigg

    MrTwigg Well-Known Member

    Yes, I did a search, several in fact and I'm not learing what I need to know. :confused:

    I have a Colt A1 post ban 20 " upper, it's marked CM on the carry handle and MO5 on the front sight base. The only other marking is a 0 on the underside of the barrel near the receiver.

    I understand all Colt barrels are chambered 5.56 and not .223 - at least from searching here.

    There are no other markings on the barrel or upper receiver.

    My questions are:

    Is the barrel chrome lined ?

    What is the twist rate ?

    What is the best bullet weight for whatever twist rate this has ?
  2. FlyinBryan

    FlyinBryan Well-Known Member

    the cm most likely stands for colt martin marietta.

    martin mariette is a company that has made parts for the aerospace industry and i believe they made some receivers for colt.

    the barrel is a good question. possibly olympic???

    you can determine the twist with a rod patch. just measure the distance for 1 full turn. its most likely 1 in 9, or 1 in 7.

    55g bullets will work pretty good in either
  3. Runningman

    Runningman Well-Known Member

    Your terminology threw me for a minute. Your Colt A1 upper and barrel is a pre ban. It sounds like is also laking some markings so it was most likely a surplus M16 upper at some point if it has small receiver pin holes.

    Yes the O on the underside of the barrel near the receiver tells me that it is chrome lined. Colt marks some barrels that way.

    All Colt 20" A1 barrels are 1 in 12 twist.

    40 to 55 grain bullets will work well in it.
  4. Runningman

    Runningman Well-Known Member

    Just wanted to add that your set up was most likely made in the late 60s.
  5. MrTwigg

    MrTwigg Well-Known Member

    Yeah, I thought it was a post ban as there is no bayo lug. (Coulda been ground off)

    I was checking the stickies and googleing around to check the barrel profile and it dosent match anything I've found so far.

    The BCG is definatly Colt also. The whole upper is in really good condition and I have a vortex style four prong flash hider I need to put back on. (It came with a brake.)

    Thanks and a Tip O'the Stetson™ [​IMG]

    I knew I could get the info I needed here.
  6. Runningman

    Runningman Well-Known Member

    Humm that could be a problem. Do you have calipers or mike to measure the barrel diameter in front of the FSB. It should be .565 - .575 if it is a A1 barrel.
  7. MrTwigg

    MrTwigg Well-Known Member

    Yeah, I'll check, I'll also check the twist rate as suggested.
  8. vanfunk

    vanfunk Well-Known Member

    How 'bout posting a few pics? We could say with much greater certainty what you have there, if we could see it.


  9. krs

    krs Well-Known Member

    I have what I've always understood to be a Colt A1 upper with a 1 in 12 twist, .5725" diameter, and marked "C MP CHROME BORE" barrel. The only marking to the upper are "C H" (the 'H' is slightly lower than the 'C') in the depression below the AI bulletnose rear sight adjuster on the right side of the carry handle. It's got a cast in 'C' on the right side of the front sight which is held on with solid pins and does have a bayo mount.

    I brought this for comparison help but my upper uses .2430" pins, small pins which always sort of made me wonder where it came from too.

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