How is it determined where on the barrel to install express sights?

You'll get WIDELY differing views on this, along with WIDELY differing photos of original express rifles with WIDELY differing placements.

The best placement from an optical perspective (pun intended) would be to place the barrel's open vee sight as far forward as practicable, usually somewhere close to the position of the forward hand or the end of the forearm. This allows the best compromise between the two focal distances (sharper sight picture) while maintaining enough distance between the sights to ensure accuracy. IOW the shooter's eye can more-quickly and more-accurately align the sights, therefore the shot placement will be faster and more accurate.

As shooters age and vision changes, this further-forward placement of the open sight becomes far more accurate and comfortable for the shooter compared to having the sight placed back at the chamber end.

A peep rear sight OTOH should be placed as far to the rear as possible while still being far enough forward so as to be somewhat protected by the body of the rifle.
JMOFWIW, regards, Joe

As JD alluded to, we have hashed this out here on the forum in the past.

Personally, I really like the "look" of an express sight placed back toward the receiver. But, like many things I find aesthetically pleasing, this is not the best set-up from a utility standpoint.

As Steven Dodd Huges pointed out +- 25" forward of the butt is a good starting point when trying to decide where to place the rear sight.

I hate the look of a rear sight positioned near the end of fore-end, but that seems to be the best place for actual use.

The great Pondoro Taylor in his African Rifles and Cartridges book covers this very thing. He was finding his eye could not focus as clearly on the rear sight on his double rifles as they could on a bolt action. He eventually realised that the rear sight on a double was invariably too close to the breach.

He goes on to say that the simplest way to find the correct position for the rear sight for your eyes is to make up an imitation one and slip it along the double rifle rib (or bolt action barrel) until you get a clear picture. That is where your gunsmith should then mount the rear sight.

If your eyes are such that the rear sight would be ridiculously close to the front before a clear picture is obtained then I guess it is time for a scope for precision shooting or a peep sight.

If you're concerned about accuracy (or rather lack of it!) with a shorter sight radius, then please consider the fact that handgun shooters commonly achieve 2 MOA & better accuracy from their HAND-HELD pistols using iron sights only & sandbag rest, no machine rest and no scope.

One of the major reasons for this is the fact that the 2 sights are in almost the same focal plane and so the shooter's sight picture is much clearer & sharper than with a longer sight radius. IOW it's a LOT easier to center the front bead or blade in the rear notch in an accurate and consistent manner when the sights are both sharply seen.

As with other things in this life, the best-LOOKING option isn't always the best-PERFORMING one.
Regards, Joe

Right or Wrong I placed my sight so that the scope bell would clear the base of the sight. this let me use a 32mm bell getting more light and see better in the shadows. the thing that i didn't plan on was being able to see the sights better. Clint

quote:

If your eyes are such that the rear sight would be ridiculously close to the front before a clear picture is obtained then I guess it is time for a scope for precision shooting or a peep sight.

Lately, as my eyes get older, I have found that a ghost ring peep sight, or a traditional peep with the disc removed, don't have enough of a "rim" for me to sight through comfortably. It blurs too much and actually disappears. I now need a full disc with a small hole, OK for slow shooting but not for fast target acquisition.