I am a new Krag owner with a 1902 gun and sight.  I prefer the 1902 based on never having fired a shot.  So; take my post with a grain of salt.

I have been shooting Grands, 1903A3 and a M1A with true rear apertures. And also several 1903's and a Trapdoor all with the 'hole' in the rear sight.  The later two, do not function as apertures and IMHO, all they do is limit the field of view.The shooter has to center the front sight. In the first 3, the shooter gets aperture effect, where no effort is needed to center the front post.

Again, extrapolating from the 03 and trapdoor, folks are not enlarging an aperture with the Krag.  They are making a hole bigger. To work, the hole has to be right up in your face.  This is why you dont see aperture sights on pistols. They dont work.

Anyway, I am a new member and I hope I am not making to strong a post or misrepresenting the posts of others. I have been know to do that.