Disclaimer: Sorry if this has been posted/discussed before; looked through this thread and couldn't find any definitive info.... Please don't flame me

Are there any "issues" with using a carbine-length "F" marked FSB in conjunction with an A2 carry handle upper (i.e. front sight post height)? If so, does anyone happen to know if there is a shorter version available in this instance, or is one left to file one down? Thanks for any helpful info.

"F" marked FSB is .040" taller from C/L of the barrel to the flat between the "ears". "F" marked FSB is usually used with rifle or carbine with a flat top upper receiver. There are shorter sight posts available if you need. Try to use what you have first. If it doesn't work out, then look into a shorter post. "Try before you buy" is my motto. And not just firearms either !

you'll be fine

Yes this, going either way. In most cases it will still zero fine. In the extreme there are alternate shorter/taller posts you can get to correct the issue.

The detachable carry-handle and non-F-marked FSB technically requires a longer post, not a shorter post.

Thanks for the replies; I figured it would just be a matter of turning down the sight post a little more than normal (or at least more than a non F-marked FSB).

I wouldn't recommend it.  It's no fun trying to get fast and accurate target acquisition when you have your front sight post cranked way down due to a taller FSB.  I've been there, done that, tried to live with it and couldn't.  I will never use an F marked FSB on a fixed carry handle upper again.

Buy a standard A2 FSB, send it to ADCO for installation. This is what I'm doing.

DON'T GET YOU BRITCHES IN A BUNCH, ABOUT USING AN "F" FSB WITH AN A1 OR A2 UPPER RECEIVER !!

IT'S NOT WORTH THE HEADACHE.  AND TOO MANY PEOPLE MAKE A BIG DEAL ABOUT IT FOR NO REASON !!

If the 1mm Front Sight Post Height is a problem, I would suggest taking the Sight Post out, and taking a Jewelers File and gently removing that 1mm.  Then add a little Cold Blue, or Sight Black, and call it done.


I know that this is a couple of days old already, But, with the "F" FSB being 0.040" Taller then the Standard FSB, People have to keep it in their minds that 0.040" is only 1mm.

When yon turn your Front Sight Down 1 Click, if that equals 1/4" (or .250"), that's greater then 5mm's difference (if I'm correct).

I run an FN A2 Upper with a 16" Palmetto CHF Barrel that has an "F" FSB, that is Extremely Accurate so far out to 300 meters with Open Sights.  I have not been able to go out any further, due to range restrictions.

I will be going out into the desert to do some distance shooting when it gets a little cooler, for longer ranges.  I'm hoping that I'll be able to keep my shots on target out to 600 meters.

Sometimes I wish I still lived in Az so I could head out to the desert and let loose some rounds. I used to go all the time in the late 80's, early 90's. Last I was out for a visit around 02, or 03, it seemed that the desert had receded from Phx by quite a bit.

I have a Colt 20" government profile barrel with F FSB on an A2 upper receiver.  Just filed .030" off the front sight pin for it to sight in at 100 yds. with the pin base flush with the top of the FSB.  Shorter front sights come in any length…if you own a file!  

Interestingly I have an FN 20" non-F barrel on an FN A2 upper receiver and had to do exactly the same thing, .030" off the sight pin for it to be flush when sighted in.  

Have some flattop upper receivers with non-F FSB and I used a .040" taller sight pin from Windham.

On a carbine you need an F marked FSB. They were originally designed because carbines required a taller post,  if it is F marked you can use a standard post.