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Seems like the upper or lower is out of spec. Try putting the Colt upper on one of your other lowers
and one of the other uppers on the Colt lower. You will then find out which parts are out of spec.
Then call Colt and ship the rifle to them to be fixed and let them know what you found out.
I can tell you from experience if the detent pins are poorly machined it's a pain trying to close/open the takedown pins on the lower.
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It seems like the lower is the problem. I swapped with three other ARs from three manufacturers, including a pistol, midlength and carbine. This is what happened.
The Colt upper fit on all the other lowers without a hitch. All three uppers from the other ARs had the same problem with the Colt lower. I have a feeling that the pivot pin hole and takedown pin hole are just a little further apart on the Colt lower than the other lowers. Here's why. If I put the pivot pin through the upper when the upper is on the lower at a 45 degree angle, then no problem. It goes in just fine. Then the takedown pin requires a hammer and punch and I have to hold the upper pressed down against the lower with quite a bit of force. If I join the upper and lower and put in the takedown pin FIRST, then it goes in just fine, but now the pivot pin will not go in. I have to push the upper back into the buffer with a lot of force and use a hammer to get the pivot pin in all the way.
This was true with all the uppers on the Colt lower. Tight is great and all that but, damnit, I don't want to have to use a hammer and hold an upper and lower together every time. It's starting to leave marks on a brand new upper and lower.