That is if the gun is actually sighted in at the factory at all. A lot of guns are spec'ed tight enough that they do not need a lot of regulating, besides, the factory has no idea how or what you are shooting. Some guns come from the factory a mile off.
Whether a sight is fixed or adjustable does not affect what range you can shoot it at, all other things being equal. We shoot at stuff 50-100 yards away with fixed sight snubbies and can get occasional hits and put the rest close. A 3 inch .357 should be good out to 25 yards for the average decent shot I would reckon.
One thing to consider is that at short ranges, your gun will shoot lower because your sights are mounted some distance above the bore line (as an extreme example, think about shooting an AR15 rifle with the muzzle touching the target. With your sights on the bullseye, your rifle muzzle will actually be touching the target a couple of inches low because of all the distance between the sights and the barrel). Fixed sights are usually closer to the bore line than adjustable sights and often do better in this area (assuming they are sighted in).
Your "point blank" range is the intersection of you sight line and your trajectory. If you shoot at the close end of this range, your bullets trajectory will be below your sight line. Chances are it may shoot much closer to point of aim at 25 yards.
Different bullet weights, and velocities can cause different points of impact. For a serious carry gun, we used to shoot a couple of different kinds acceptable ammo and take the one that was closest. Most of the time, especially with nice consistent quality ammo and a decent gun, any load was close enough. I would consider 1-2 inches low at 15 yards close enough. If it was too far off, then the files or the barrel vise would come out. Sometimes different grips, a different stance, or single action or double action would change the impact as well. How well you handle recoil seems to make a difference too. If I remember the formula to calculate recoil was a function of bullet weight, velocity, gun weight, and weight of powder.
For a lot of people, it was enough to know about how far off it was and use kentucky windage. This technique worked well for the one-gun person (not me).
I would say just shoot some loads at different ranges (say 15 and 25), pick one and remember whereabouts it shoots. If you are only off a little vertically and are good horizontally, you are in great shape! adjusting for windage on a fixed sight gun really sucks!
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