Are "F" front sight bases taller or are the sight posts in the bases taller?

Are they really necessary?  I wonder because it seems like most companies are not using them.

Is this also required on 20" barrels or just on the carbines with the short sight radius?

Thanks guys.

The differences in the FSB is that the 'F' base is milled between the ears to create a taller platform for the FSP...

The circled areas are where the differences are...

The base itself is taller, there are taller sight post available, but they are not really the best silution.

During developement of the A2, Diemaco was working on the C7... they developed the C7FT that used a flat top rail and detachable handle/sights that worked with the standard front sight -- however, when the US version of the upper was adapted to use the 1913 Picatinny rail standard, the height of the rail surface was raised slightly... to make up for this, the taller front sight base should be used on any rifle that uses a flat top.

Here's the link that can explain it all or just confuse the hell out of you...

www.ar15.com/forums/topic.html?b=3&f=118&t=251786

As shown in the pictures above the "F" FSB is machined as to allow the front sight post to sit higher.  When Colt's put the flat top upper and carry handle into production in 1992 their handle placed the rear aperture a smidgen higher then A2 upper receiver did.  I can only tell you what and when, don't know why Colt's did this.  Sometime after 1995 Colt's realized there was a problem with some flat tops not being able to zero due to the rear sight being higher, so they introduced the "F" FSB to correct this.  From that point on Colt's flat top Carbines, and Colt's/FN flat top rifles use the "F" FSB.

Non USGI AR makers are different, don't know the story on them.

Colt made some proprietary design changes, which were patented and incorporated into the specs. When the Army released the Technical Data Package out to bidders, Colt sued the government to keep it proprietary to Colt. This left Colt as the only parts maker, as the design data was not available to the industry.

While everyone had seen the data, they were legally prohibited from using it. That is when the rest of the industry went with the A2 specs and Colt went with the M4 specs.

Quoted: Colt made some proprietary design changes, which were patented and incorporated into the specs. When the Army released the Technical Data Package out to bidders, Colt sued the government to keep it proprietary to Colt. This left Colt as the only parts maker, as the design data was not available to the industry. While everyone had seen the data, they were legally prohibited from using it. That is when the rest of the industry went with the A2 specs and Colt went with the M4 specs.

What happened with the TDP is that RIA gave Crane a copy in 1996.  Crane turned around and sent the TDP out to contractors to solicit for some SOPMOD stuff.  One of the outfits Crane sent the M4 TDP was FNMI.  Colt's also got a copy, and found out Crane had sent a copy to FNMI.  Colt's protested to RIA.  RIA then informed Crane they needed to get those copies back ASAP.  Colt's then announced that the US Government's license for the entire M16/M4 was terminated for breech of contract.  And the soap opera went on from there.  

But it was FNMI that sued, the case was "FN Manufacturing, Inc. v. United States", in order to be able to bid on the M4, and was shot down and it ended up with Colt's getting a "sole source" deal.

www.ffhsj.com/govtcon/ffgalert/gcarch/nb981206.htm

Any how, my point is that this all happened in 96-98.  Other makers were making flat top uppers and carry handles prior to 1996.  So I am unclear on how the 96-98 soap opera has anything to do with after market flat tops and carry handles when they were in production prior to this.