I have a Sabre mid-length upper with a Troy BUIS. My front sight base is marked with an “F” but the front sight post is cranked up pretty high. I’m trying to figure out what is wrong here; is it the front sight base or is it the Troy BUIS?
Did Sabre use a standard FSB and just mark it with an F or does Troy build their BUIS to match the original height of an A2 upper?
Have you tried measuring the height of your front sight base?
GotM4 here has a picture indicating the height difference between F-marked and standard FSB.
We have seen a lot of F marked FSB’s that are too short. You might have gotten a bad one.
C4
Not my pics, I found them on another website.
My Troy’s work with Colt F marked FSB’s on M4A1’s (real ones), LMT’s, KAC, and CMMG F marked FSB…
I installed a Troy BUIS on my Sabre Defence M4 a couple of weeks ago and works just fine.
Thanks guys, I’ll try and measure it to see what’s up. I suspected that it’s just a normal FSB marked with an “F” but I guess the measurement will tell me for sure.
I’ll let you all know what I find out.
Proper credit for those pictures goes to Molon on Arfcom.
I use those pictures as well. 
You using pictures? No way!
Thanks to the pics I think I understand the difference between standard and F-marked front sights.
Why is there a difference?
Are A1 and A2 handle attachments usually made to replicate the original receiver, or to suit F-marked
front sights? Is there any practical difference?
ETA: Post Deleted, my browser refreshed but failed to show replies and answers to your question until I submitted my reply.
The top of a M1913 receiver is thicker than a standard receiver, which increases the height above bore of the rear sight. The front sight has to be raised to match.
The hoop on an A1/A2 should add rigidity compared to a flat-top, so it makes sense that a flat-top upper would be thicker.
Why wouldn’t the difference be taken up by items to be mounted on the flat-top? Why mess with the front sight?
Maybe I’m deluded but it seems this system has remained pretty consistent over the years. Some changes, such as rifling twist, are pretty easy to comprehend. This front sight business puzzles me – if there’s a difference that makes a difference, there ought to be a good reason for it.
It’s not a flattop thing, it’s a carbine barrel sight radius thing.
Even earlier carbines built on A1 type receivers had taller front sights when the sight was moved back to the carbine location.
What front sight post does an M16A4 use?
I have Tory BUIS on my Sabre mid-length and I was able to get it zeroed at 50 yards without any issues. I built my upper and installed the barrel to a CMT upper receiver. Could this issue also be caused by an out of spec upper receiver rail?
If your FSB is indeed shorter than a F marked FSB should be I think the easiest thing to do would be to get a longer front sight post.
Keep us posted on how it goes.
I turned my A2 into an A4 by adding a Colt flat top upper. Still has the standard FSB and it zero’d perfectly with the front sight flush, rear bottomed out on 6/3.
Are we talking sight posts or sight bases?
I had a Colt 20" flat top HBAR that came with an F marked base. And I also have an 11.5" Colt A1 upper (with m4 feedramps 
) with an unmarked FSB.
I don’t know the answer, but these are two examples that I’ve seen first hand.
You can always order the taller front sight from Brownells if you can’t get your upper to zero. I had to do this with a Bushmaster upper, but it had the standard height front sight base.
The reason I resist doing this is that the higher a front sight sits in the FSB, the more prone it is to being damaged.
I’ve had to replace dinged front sight posts before, so I feel warmer when they sit lower in the base.
You must not realize that an extended post in a standard A2 FSB sits EXACTLY the same height as a standard post in an F FSB does.
The height of the ears in relation to the post is the same in both cases.
The same forging is used for both A2 and F front sight bases.
It’s only the height of the shelf that’s different.
An F front sight base has shorter “ears” than an A2 does.