What markings on the FN M16 FSB ID's A2 vs. A3/A4?

Thanks in advance.

First, FN built A2s, and was the only supplier for A3s to the Navy. Both those models had carry handle uppers. A4 has a rail, which needs the F marked FSB.

I qualified w/ carry handle A3s, but later contract A3s are flat tops.

I have both.  The only difference will be the A4 barrel will have an F marked FSB and the A2 will not.  Otherwise they are identical and marked the same.  Some will have FNMI on the barrel and some will have the CAGE code 3S769.

A4 barrel with F marked FSB




A2 barrel with A2 height FSB

Just because the FSB does not hav an F marking does not mean that it isn't "F" height, especially earlier A3/A4 uppers.

Of course, you should not see an F marked FSB on an A2 rifle.

I have recently observed many FN and Colt A4 uppers lacking an F marking on the FSB in the MI Army National Guard arms rooms.  I am not sure if the sight base on these uppers are "F" height and/or if the height is redundant on rifles with 20" barrels with the longer sight radius.   All of the M4 rifles I observed have F marked FSBs.  

Also, after observing and measuring a number of modern commercial FSBs, the sight bases were all taller "F" height.

This is purely conjecture, but with A2 uppers pretty much falling off the map, the taller FSB sight base has symy evolved as the standard height.

The FN M16A2 barrels I have owned are marked FNMI on top of the barrel. I know some are marked 3S679 instead - I assume those are later production.

The A2 height FN FSBs have a raised circle with an FN in the center - on my barrels this has been consistently on the right side of the FSB.

There is usually some grey paint daubed on over the ends of the FSB pins.