Thanks everyone. I'm understanding better now. I found a copy of TM 9-1005-249-10, the operator's manual for the M16 and M16A1 that I bought at a gunshow years ago. It shows there that those models had a rear sight with two apertures for range (page 2-16). The unmarked aperture was for targets from 0-300 meters and the aperture marked L was for 300-400 meters. The low light level system on the next page indicates that the apertures of that different system have different functions: one being simply a low light ghost ring style and the other to be used for normal conditions.
So with the standard sights on the M16 and M16A1 you'd switch apertures to the L aperture for targets beyond 300 meters, the L meaning "long". Sighting in would be done, it looks like (page 2-19) using the L aperture for point of aim at 25 meters, which would put the battlesight zero at 375 meters with the L aperture and at 250 meters when using the umarked aperture. Using the unmarked aperture, the trajectory would never rise above 3.5" and would be at point of aim at 250 meters. The bullet would impact below point of aim before 25 meters and even a short distance beyond 25 meters (since it was the L aperture that was used for the 25 meter zero) and also beyond 250 meters. This, of course, with the ammo and twist rate of those models. For the faster twist rates and the new varieties of ammo we're shooting, trajectories would differ.
Okay. So now I'm wondering how the LaRue and Troy dual apertures are set up. Do they follow the purpose of the old A1 standard sights, where you switch apertures for distance, or are they designed like the earlier low light level system mentioned above and on page 2-17 of TM 9-1005-249-10 where one simply allows more light but doesn't change point of aim?
What's got me wondering is Troy's website saying of their folding battle sight: "Dual peep, same plane aperture allows for perfect co-witness of red dot and holographic optics." This sounds to me like the apertures do not affect point of aim but only differ in amount of light allowed. LaRue doesn't go into detail. I've never used peep sights so I realize I may be misunderstanding.
As far as which of the two I'll choose (LaRue or Troy) I'm still leaning towards the LaRue because I don't think I'll be getting an Aimpoint or EOTech, or using a scope, for quite a while, so the rear sight isn't really going to be a Back Up sight, but my Main sight.
GL
+++ That money order for my Bronze membership must not have arrived at AR15.com yet. +++