It would be nice if there was complete verification on this issue. From what I know, and I could be wrong, when colt builds a rifle with a flattop upper, the M4 front sight base is used along with the M4 carry handle, no matter what the barrel length is. When the flattop concept was being developed, it was decided to make the sight plane higher, increasing the height of the carry handle and front sight. As for the reasons for this I am not sure, maybe to give the elevation post in the handle more room to move, or make the carry handle opening larger so that you could still sort of use it. Whatever the case, when the other manufacturers starting building rifles with flattops, they decided to just stay with the original front sight and design a lower carry handle.