Here is the model that I have ordered and will be testing in the upcoming weeks.

The concept behind this is that the human eye has an easier time centering something in a circle rather than between posts.
This concept has been embraced by the long gun community through the use of peep sights / ghost rings / even scopes are round.
If you have ever shot an MP5 with its barrel rear sight you know how easy it is to acquire a target and get rounds on target accurately.
It seems logical to me that if you have a ghost ring rear sight on a pistol and you are using a two eyes open combat shooting sight picture as long as the target is in the rear circle you will be relatively accurate at normal gunfighting distances.