I can’t find this anywhere here or on the web, so if you know of a thread that already answers the question, feel free to post a link to it.
So…Colt 6720…is the front sight “F” height or not? I have a hard time believing it’s not. All my tools are packed up and I’d like to know. Also, for educational purposes, can someone tell me what the existing markings mean? Is this just a Colt thing and is it common to all their rifles?
I would imagine all rifles and carbines from Colt and other reputable manufacturers (BCM, DD, LMT, etc) provide F marked FSBs for their flat top rifles and carbines. There should be a F somewhere around there, either stamped or cast, or perhaps on the other side of the FSB.
Concerning the markings, I think it would be safe to say that only Colt knows exactly what they mean. If someone else does, I haven’t seen it in my research.
Nope. No “F” anywhere on there. I’ve been finger-banging this thing for some time now while I wait to go to the range, so I’m pretty sure I would have seen it. 
Colt is well-known for being GTG, but it would be nice if someone has measured a rifle with these markings and can confirm the height. I have a 6520 around that has a C and a 6 on the FSB. No idea what the 6 is for on that one, either. 
I began to think this was a stupid question, but it’s occurring to me now that the lightweight barrel should actually measure differently from something like a 6920. And a 6520 would be yet another variation due to the fixed carry handle. I guess I need to start unpacking my crap, measure this sucker, and report on how the zeroing goes. I guess in the big scheme of things, the 6720 is kind of an oddball for a Colt factory carbine. If anybody wants to measure FSB height on a BCM or similar LW rifle, please do so.
LMT has an F height FSB, but isn’t F marked. Mine is marked <B>1 and I wouldn’t be surprised if yours is F height too, in fact I’d be more surprised if it wasn’t, it’s Colt after all.
6720s do not use an F marked front sight base nor is it F height.
Just curious…
From the bore centerline, are the heights to the top of the sight base ears (for lack of a better term) on the 6720 and a standard F base the same?
I realize that the bases would have different physical dimensions due to the reduced OD of the barrel, I was just curious if, overall, the front sight is at the same “height”.
If this question isn’t worded clearly enough, I apologize in advance.
If I remember correctly, the 6720 just uses a taller front sight post to allow for zeroing.
C4
Are they using the post off of the 9mm SMG?
Don’t think so.
Good to see you on here Tweak!
C4
was asking because last I knew Colt had only the 4 posts, the round and square rifle, the taller 9mm (which may have been created to use with flattops before the change to F bases) and the tapered carbine. A photo of the front sight base showing the relationship of the tip of the FSP to the ears of the FSB would be telling. 
good to be here, seems to be a little more technical than some other places. 
They are:
All post 1984 USGI AR-15s use the same sight post so they are interchangeable between all guns. The 0.040" is compensated by using a F marked FSB on all flattops. Thus an armorer/soldier cannot put an incorrect post on a gun because they are all set up to use the same post and there is only one post in the supply system.
Most Colt commercial/export fixed handled carbines the different height posts to compensate for the 0.040" difference.
Using a short FSB with a tall post is the same as using a tall FSB with a short post. Thus the 6720 has correct height sights for a flattop AR-15.
Being “technical” is what M4C is all about.
C4
Nice post.
Thank you. Do you have the Colt PNs for those two?
Comparing the posts between my 6520 and 6720. The posts seem to be the same height. The one on the 6720 is just run up higher in the front sight base {about a 1/32"} than the one on the 6520.
I don’t. I just rob them out of Colt barrels that come my way as they are very difficult to find.
They should be the same height.
My understanding is that the 6720 uses the same fsb as the 6520. Here are pictures taken tonight of the 6720 and 6920 fsb/post respectively. The base on the 6920 is marked “F,” and the 6720 is not. I bring home some good calipers from work and measure.
Finally measured the thing, and it’s 2.010 from top of barrel to the flat top of the FSB. I believe this was pretty much settled, but I thought I’d throw some numbers at it for posterity.