"Sight radius" is the "effective distance" between the front and rear sights, not the distance between the center of the sight mounts. The longer the effective sight radius is, the more parallax error will be reduced (parallax error is the visual problem one has when attempting to accurately align two or more independent points such as a front & rear sights and a target) As for the sights themselves, the "best sights" are the ones that work best for you and I strongly suggest you familiarize yourself with all the options available before making a decision. Sight radius is maximized by utilizing as long a barrel as possible and choosing sights that are not only properly matched to the application but also best utilize the available length.
Here's an example: This is one of my slimline underhammer rifles with a 36" long barrel.
The "Green" line indicates the sight radius for the typical traditional sights which on this one is 26.5"
The "Yellow" line indicates the sight radius for a typical receiver/action-mounted peep sight which on this one is 35.5"
The "Red" line indicates the sight radius for a typical Schuetzen/Creedmoor style sight which on this one is 38"
Take note, the shortest sight radius on the above rifle is longer than most modern rifle barrels so if you look at a typical 22"-24" long modern rifle barrel that has barrel-mounted OEM sights, the sight radius is usually around 14"-16" at best. History has taught us what works and what doesn't when it comes to sights and it seems as though the vast majority of modern gun mfg's either haven't learned anything from history, haven't bothered to study history and/or just blatantly ignore the time-proven facts. Invest a few minutes trolling through some on-line forums and you'll find posting after posting by shooters claiming the absolute "need" to put an optical sight (scope) on a rifle or handgun. Such comments do not surprise me any because the vast majority of modern guns are fitted with sights that suck. Yes, I said "suck" and I stand firmly by that statement because the evidence to support the fact is overwhelming.
As the available sighting area becomes smaller, the importance of utilizing sights that make maximum use of the allotted space and that properly match the intended use becomes increasingly important. Having studied the feedback from hundreds of shooters and working one-on-one with dozens of shooters, it became readily evident that the common complaints expressed by modern day shooters are the same complaints addressed by shooters throughout history. For example: One of the most common modern complaints is the inability to maintain consistent accuracy beyond a given range when "shooting over iron" - more often than not, this problem is primarily caused by the combination of a sight radius that is too-short and front sight blade/bead that is too-large and thus causes increased sighting error as the range is increased.
