Hello all,

I was wondering the other day why Colt does not make a flattop carbine utilizing
the government carbine (6520) barrel. SO, I decided to make one for myself. I notice
that the 6520 barrels don't have the F-marked front sight. I was wondering what the
implications are of putting the 6520 barrel on a Colt M4 flattop upper. I know there are sight height
differences, but will this combination work together? Do I simply need to get a taller front
sight pin? Or what?

Thanks in advance for your kind responses,

Mark Papineau
Loveland, CO

I would doubt that you'd need a different front site post on a flat-top. . .

Quoted: I would doubt that you'd need a different front site post on a flat-top. . .

Well, you know, I mean with the Colt carry handle mounted, will I have enough upward travel in the standard front sight? Sorry if this was confusing.

Mark

Mark, I'm in Fort Collins. Let me know if you ever want to talk AR's. I have some old AR10's too, if you're into those.

Actually, the flat tops have the different height front sight bases, but heck if I can remember the heigh differences. I'd suggest doing a Search over in the AR15 forum, either Uppers or the one for Sights, or General Discussion.

OK, I did a little research and it seems that there indeed two front sight bases.  These sight bases that are marked "F", for Flat Top, are .032" taller that the A2 models.  Apparently, many non mil-spec. ARs come with the standard "A2" FSB, making it hard to get a proper zero with mil-spec rear sights and BUISs.  

I know that the Colt carbines come with the "F"-marked FSB, I just looked at mine.

Below is an image of one that Bravo Company U.S.A. sells:



The "F" on the Colt FSB is smaller and to the left of the "F" on the above base.

interesting !!  that "F" has only been there since Colt made their first flattop !!

For what it's worth, my AR6721 tactical carbine came with an "F" marked front site base, as well as my MT6400C M4 and my LE6920 upper.

Like most people I'd rather use the correct FSB than raise the front site post for zeroing. Once the post is raised on the shorter FSB the tip is nearly clear of the ears, so it's unprotected.