It is generally acknowledged that a 20" barelled AR has a 20" sight radius. My contention is that it actually measures from
front sight to eyeball- more like 24". In the past I've seen reference to using the front sight as a rangefinder and the math does come very close if I use the 24" figure- at least for
me. Please comment.

As far as I know, sight radius is defined as the distance between the front and rear sight – it has nothing to do with rangefinding.

Obviously, if you’re using the front sight as a rangefinder, the distance to your eye determines the angle the front sight subtends and is certainly important.  However, in that situation the location of the rear sight is not important – indeed, you don’t even need it.

Also, if you’re using the front sight as a rangefinder, your eyes are a heck of a lot sharper than mine!!  [:D]

If I read you correctly, you seem to be mixing two different issues.

Well I'm not ranging golf balls at 600 if that's what you mean. But the point I was trying to get at is ,on aperture sights the radius is much longer than on traditional sights and perhaps this is one of the reasons that I had so much trouble with 25 yard sight-in
What I finally did was accept the fact that (at
least on the AR ) I just accept a 250 yd. zero
and aim dead on with no holdover out to 300.that
means no more than 3 inches above or below the
line of sight. Since my house range only goes to
125 it meant a 90 mile drive to actually test this, and there were many wasted trips.