When talking about the distance between the front sight and the rear sight of the rifle, it seems to me "plane" is the appropriate term, but I see a lot of people use "radius" instead.

So which is it?

I say radius, but then again, I also say tomoto. bored, no gig tonite

I've never heard sight plane used to describe the distance between the front and rear sight, since plane usually refers to a two-dimensional flat surface that extends in four directions. I've always heard sight radius used for it, which I guess made sense to my engineer brain since radius is usually just a one-dimensional distance.

If the distance is 24" between the sights, is that a 24" plane? I have never heard the term plane to describe a distance. I guess I've never given it any thought.

http://www.m-w.com/cgi-bin/dictionary?radius
I think #5 applies here.

I've generally heard of the "sight plane" being the plane that contains the line of sight and the centerline of the bore, with the "sight radius" being the distance within that plane from the rear sight to the front sight.