Quoted:
Generally, yes.
The rear sight base is the same.
The carry handle is different.  .793" on a rifle.  .832" on a carbine. { Depth behind the drum}   About .040".
Remember, all those threads about needing  a FS that was .040" taller?
Mis-matched FSB and carry handle may have caused this.
458

Quoted:


So the carbine carry handle is the one like the first pic?

Quoted:
Doesn't the use of an F-marked front site base depend on the rear sights on the detachable carry handle? There are two different kinds:

One has the rear sight sitting below the top of the carry handle


The other has the rear sight sitting above the top of the carry handle


Which one requires the use of an F-marked FSB? Thanks!

Quoted:

That's a good question - I never knew there was 2 different versions of removable carry handles as illustrated above

I'm also curious to know what the difference is?

Sakic #19

Quoted:
They differ in height behind the drum.  All other dimensions appear the same. IME.
458

Quoted:

BCM - Bravo Company MFG, Inc.

Mil-Spec height carry handle for flat top AR-15, M16A4, M4, M4A1 rifles and carbines.  Compatable with taller F marked front sight bases.

Machined from 7075T6 forgings, hard coat anodized, staked thumb screws, A2 aperature, 6/3 windage drums.

Quoted:
I know the difference.

Do you
458

Quoted:
As far as I know MIL-SPEC carry handles do not come in different heights.  To try to confirm my thoughts I have been trying to find a vendor who offers a carbine carry handle and a rifle carry handle and I can't.

The "F marked" FSB is made to correct for the difference in height between the sights on a flat-top receiver with MIL-SPEC carry handle and the sights on an A2 upper.

Below is the description of Bravo Company's Carry Handle copied and pasted from their website.



Obviously something is different about the two carry handles pictures but I can't figure out what it is.

Quoted:


So the question now is which one of the two pictured above is mil-spec.

Quoted:
"No idea from looking. It seems I may be wrong anyway after reading 458's post (one of us is wrong, it might be me). Maybe the rifle and carbine heights are different, its news to me if they are though."  

You're not wrong.  But I may be.  My experience and carry handles on hand may be all.. Wrong.   Might want to measure yours.  They might be wrong too.
458

Quoted:
OP - The taller one is the "mil" version.  If it is the one you have you should use the "F" FSB.

tall carry handle + "F" FSB = good
short carry handle + reg FSB = good
short carry handle + "F" FSB = ok
tall carry handle + reg FSB = fail  (sometimes they work – sometimes they don’t.  Use a taller sight post if necessary)

It was the thickness of the top of a flattop that created the problem, not the barrel length.  Colt realized it and switched to the “F” FSB.  Other manufacturers realized it and switched to the lower carry handle.  Two different solutions for the problem created the confusion.

Quoted:
The difference between the "F" marked FSB and the old standard FSB is that the "F" marked FSB is machined to allow an additional 0.040" of upward travel of the sight post. This is to compensate for the change in sight geometry due to the shorter sight radius of the carbine length gas system vs. the rifle length gas system. Most carbines do not need this extra sight adjustment, but it is nice to have. In the days before the M4, Colt (and some other manufacturers) carbines had a 0.040" taller front sight post installed. When the M4 was adopted by the U.S. military, it was decided that it was easier to machine the FSB for more elevation travel and use it on both rifles and carbines, than it was to have two different height front sight posts in the inventory system.

Many people mistakenly believe the "F" stands for "flattop", since the flattops came about around the same time as the "F" marked FSBs. This just isn't true. The "F" mark could have been "Z" or "K" or "T" or any other symbol. It does not stand for "flattop". The rear sight on military standard flattop carry handles are the same height at the lowest setting as the lowest setting on an A2 upper. (Though the max height is shorter due to the elevation screw being shorter to make room for the flattop rail.) Much confusion results from certain commercial manufacturers deciding to make their flattop carry handle 0.040" shorter so that they wouldn't have to keep two different height front sights in inventory, and they didn't have to change their tooling for their FSB, or buy"F" marked FSBs from govt. contractors. At least one such company advertised at one time that their detachable carry handles didn't require the "F" marked FSB.

The carry handle at the top of the OP is one of the non-milspec commercial ones.

Karl

Quoted:
The commercial carry handle was the part that confused everything.  Bushmaster (the top picture in the OP's post) shortened theirs to accommodate the regular FSB on flat tops.  As stated in the above post most companies use the mil spec carry handle with the "F" FSB on both rifles and carbines.

I was of the opinion that the "F" was just a letter and had no significance, however, recently another arfcom user posed the question to Colt.  In summary the question asked was does the "F" relate to the length of the carbine barrel or does it go with the flat top.  The answer was that "F" does in fact stand for flat top.  The thickness of the top of the upper receiver caused the carry handle to sit a little higher than a regular A2.  The allowable tolerances could make it so some carbine front sights need additional height to lower the POI.  Therefore Colt introduced the "F" FSB which has a taller shelf.  Bushmaster lowered the carry handle to solve the problem -  two solutions for the the same problem.  It is my understanding that "F" FSB's are now used on both rifles and carbines.  This can be done because the "F" allows more upward travel in the sight post, but there is still plenty of room for downward travel because the post can screw into the base below the FSB shelf.

I still believe the barrel length (or where the FSB is located) has some impact on the entire equation, however, that impact would have been acceptable if the flat top's top thickness didn't raise the rear sight.  So, yes the carbine impacts the front sight, but it was the flat top that created the need for the "F" FSB.

Quoted:
Correct, there is no need for the "F" on a rifle.  A regular FSB will work, but so will an "F".  Although I don't have one, according to Colt the current M16A4 rifle uses the "F" FSB - it's easier to standardize the same parts for everything.

Here is why the both work.  The tolerances in the flat top carbine with the regular FSB were close, sometimes they worked, sometimes they didn't.  The taller shelf "F" made sure they worked all of the time.  If you move the FSB farther out to rifle length you would have to screw the sight post down into the FSB to raise the POI.  There is plenty of room in either sight base to screw the sight post down to accommodate this.  So their was never an issue with the regular FSB on a flat top rifle.  But, it's easier to keep them all the same and use the "F" on the rifle, too.

Remember these are for military applications.  Civilians have the luxury of purchasing and changing parts if something isn't right.  Mixed up receivers and FSB's can always be corrected with a taller sight post, etc.

I hope this isn't confusing you.   For your original question you should use the "F" and it really doesn't matter what "F" stands for.