Yet another Colt AR question...

[MrTwigg]

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

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 ?

 

[FlyinBryan]

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

 

[Runningman]

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.

Is the barrel chrome lined ?

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.

What is the twist rate ?

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

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

40 to 55 grain bullets will work well in it.

 

[Runningman]

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

 

[MrTwigg]

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™ View attachment 432990

I knew I could get the info I needed here.

 

[Runningman]

there is no bayo lug

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.

 

[MrTwigg]

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

 

[vanfunk]

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

Thanks!

vanfunk

 

[krs]

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.