As I understand it, in order to zero your iron sights with a flattop Colt you must have an "f" marked front sight base. What if you have a fixed carry handle and an "f" marked front sight base? Thanks in adavance-

Then you might have to get the taller front sight post which is sold by Bushmaster and others.

Then again unless you've zeroed your rifle's iron sights, you'll not know whether you have a problem or not.

Mike

the carry handle should zero fine on your colt flat top with or with out an F marked FSB, the F marked FSB is only needed to zero most BUIS, in that case if you don't have one then you will need the taller fron site post made by bushmaster...

Well, for skatyeighth time I will post that it AIN'T the flat top. It's the barrel length. Carbine type ARs almost always require a higher front sight point, whether thy're flat top or one piece uppers.  My SP1 carbine came from the factory in 1978 with a higher front sight. Rifles almost never do need the higher front point.

Up until the advent of the M4 series, Colt used the same higher sight insert on carbines and submachine guns that Bushmaster now sells, for standard bases. When the Army didn't want to keep two different heights of front sight blades, Colt incepted the "F" series front sight base, in which the screw in shelf for the insert is the same .040 higher as the old type carbines had in the higher insert.  Colt and FN have now standardized on the "F" base for both rifles and carbines, because you can always screw the sight post diown further on the higher base in a rifle, but you can't raise it past a certain point on a carbine.

So yes, if you have a carbine type AR you need a higher front sight point, flat top or one piece. Since you say that your sight base has an "F" stamped into the left side, you have the higher mounting point in the base and can use a standard sight insert.  If you have a rifle type with an "F" marked base, you would still use the standard sight blade, but will probably have to screw it down a few clicks into the base, which should be no problem.

The old wives tale about Colt flat top/sight assys being higher started way back when the flat top was a new hot item, and some of the early aftermarket pieces did have non standard measurements. Those days are long gone. All the major makers now put out flat tops and sight assys within a few thounsanths of each other dimensionally. My Colt SP1 carbine, built years before anyone thought about the "F" base, needs a higher front sight point, accomplished by a higher post.  My Continental upper/Bushmaster sight assy M4gery needs the same higher front sight point, but since the Rock River barrel has a standard sight base, I also needed the Bushie higher post on it.

Thanks! That was very informative. SO, I have a 20" match target competition in wich I am adding an A2 20" Govermnet bbl that does NOT have the "f" marked on it. Will I be ok since it is a full size 20" rifle?

Sorry, that was unclear. I wanted to confirm a  different scenario. If one has a 20" colt flattop HBAR and wishes to dump the HBAR barrel for an A2 Gov't profile bbl, will everything be ok if the FSB does not have the "f"marked? By your post, as I understand it, it is ok because it is not a carbine....
Thanks