Remember that there is a differnce between front sight bases on Colts for A2s and Flattops (marked "F").  Have a few quesitons as shopping for another barrel this weekend.

Link added to AR15 Thread on F Front Sight Bases

Question #1: Which front base is higher and by how much?  Appears the Colt Carry Handle and Front sight is 0.040 inch higher.  

Question #2: Is the front sight base differnt on the "F" marked Colts, or just that a different post is installed? - coverd in later posting

Question #3: Per posting the GG&G plus ARMs #40 are setup for the higher Colt flattop front sight.  What about the KAC 300 and 600 meter sights?

Regards
John

Edited as information was found.

You're fine with either backup sights on the flattop.

Colt M4 front sight bases are marked F, and it's suppose to be slightly taller.  Nonetheless, I have both F and non F marked Colt front tower, and they are identical in height.

The front sight forgings for the "F" marked and non-marked front sights are the same.  The "F" sights are simply not milled down as far between the ears causing the front sight pin to sit higher.

The COLT carry handles are ALL the SAME ht. Only the front sight on Colt Gov't M4's are .040 higher as it has to be to make up the dif. of a short barrel vs a long barrel.
All gov't issue mil spec rear sights are at a specified std. ht. that must work with the front sight of either the rifle or gv't ht. carbine M4's.
Good shootin, Jack