Not really sure that this strictly qualifies as an aperture sight, since it mounts into the barrel dovetail intended for the standard rear sight and thus sits further forward than a receiver- or tang-mounted sight. It's made by Marbles, replaces the rear sight and is adjusted for elevation by a stepped elevator that is just held down by the spring tension of the sight body. Windage is altered by driving it, or the front sight blade, left and right within the dovetail...just like old-style typical open sights.
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It is much more of a hunting sight than one for targets, and is much more of a PITA to get perfectly adjusted...but also very resistant to misadjustment by accident. The large aperture means it is more difficult to obtain the kind of pinpoint accuracy that a smaller aperture further rear can provide, but it is quite sufficient to produce groups at 100yd that are usually around 1.5 inches. Might do better with a scope, or a smaller aperture further back, or with another shooter with younger eyes...but might not either. It's good enough for what I want out of this gun; it's a Henry Mare's Leg with full-length buttstock. I wanted one for several years, ever since seeing pics of Shellshucker's pair of them; finally put this one together and it does not disappoint.
I have this sight on 4 or 5 other rifles as well...a Henry single shot, Uberti rolling block, Winchester levers...and I love it. The airy open construction and big aperture seems to make it more useful in lower light than standard receiver sights. It works great with either bead sights (FO or brass) or flat-top posts.
Regarding the potential danger of tang-mounted sights: I had an unpleasant mishap a few years back, using a C.Sharps single wearing such a sight. Approaching a downed animal, I somehow got the sight folded down flat, I think caused by some thick brush I had to elbow my way through. Arriving at the critter, it spooked and began to struggle to its feet. I threw the gun up and quickly got off a shot, instinctively using the barrel-mounted open sight...which resulted in driving the end of the tang sight through my cheek. Ugly nasty gash, lots of blood, really stupid-looking trophy picture, felt like an idiot.
If there had been no open sight....I think I would have not fired...maybe? But throwing the rifle to my cheek and seeing a familiar sight picture, I just followed through and triggered the shot.
What if the tang had been folded only half-way down? It would not have been blocking the sight picture, so I likely would have done the same thing...except the end of the tang would have ended up in my eye rather than in my mouth.
It spooked me badly enough that I removed the tang-mounted sights from several hunting rifles I had them mounted on. I still have a single one left, mounted on an 1886 that has no barrel mounted sight at all. I can't touch that rifle without thinking about that incident, so I'm pretty sure it will never be repeated; creeps me out a bit just writing this.