Colt ar 15 upper question

[Alex$]

Have a chance to buy a Colt National match complete upper, i remember reading there were some dimensions that were different than "standard" lowers, but cannot recall what they are.

Does anyone know what i should ask to have shown in pictures to determine the proper lower i should buy to fit the upper?

[kato4moto]

I believe many Colts used larger pins than normal (rear takedown and front one), though there are adapter pins available to mate large-pin Colt uppers to "standard" lowers.

[ciscosixgun]

On a colt the front take down pin may be a normal size or larger size, you see them both ways. That should be the only issue with a colt upper.