On my main home defense Colt LE6920 16" carbine gas I prefer just the factory standard square front sight post and I'm using a large aperture of my Knight's Armament 300m folding rear sight, I use this carbine point blank (furthest wall indoors) up to rarely 300 yards outdoors, mostly under 100 yds.
I have another Colt 16" carbine I use as backup with same factory front sight post fixed front sight but it have a large diamond shape rear aperture of my Troy DOA folding rear sight, I just prefer to use this sight combo for indoors and very short range low light conditions, outdoors on bright sunlight there's too much light going through the aperture that I have a hard time seeing the aperture.
My precision 16" midlength gas carbine 6721 also have a standard factory square front sight post on fixed sight base and I'm using a same plane round aperture (large aperture at 0 to 200 yds and small aperture up to 600 yds) Troy folding rear sight.
With my only 20" A2 rifle gas with USGI M16A2 Colt upper half its just the factory square front post sights and fixed A2 carry handle upper receiver, I use the large aperture 0 to 200 yds, the small aperture sight could be elevated up to 800 meters but I don't shoot that far, I sometimes shoot it at only 600 yds.
When I was shooting competition rifles in the late 80s I'm using smaller than factory round match front sight post and smaller target aperture with finer elevation adjustment rear sight on my Colt AR-15A2 Sporter HBAR 20" rifle.