Red Dot Sights - Parallax and Sight Error
I've read several different definitions of parallax. To me, it means that because the sight is not on the same centerline as the barrel, when you are sighted in, you are actually shooting the bullet slightly upward towards the line of sight, so its path intersects the point of aim down range. It then rises above the line of sight as it travels down-range, then starts to decend until it re-crosses the line of sight farther down range.
The only perfectly parallax free sight would have to be looking down the center of the barrel bore, and you'd need "magic bullets" that didn't drop as they travelled down range.
Red dot sights, however, are a bit more complicated. The red dot sights project a tiny red dot onto a see-through mirror, also called the "Beam-Splitter". As you move your head from side to side or up and down, you will see that the dot moves from side to side or up and down inside the sight.
I've read claims that wherever the dot is, that's the point of impact. Not so. As you move your head, and the dot moves inside the sight, it is also slightly moving on the target. It seems that the larger the diameter of the sight, and the closer you are to the target, the more the movement.
I believe that manufacturers design a certain amount of compensation into their sights, but I doubt if a red dot sight exists that doesn't show this at some distance. The 42mm. Simmons red dot sight at twenty-five feet moved well over an inch. Keep in mind, though, that a 42mm. sight is a big sight! In comparison, the C-More, which is more like 25mm., at anything over about fifteen feet didn't appear to move at all.
Here's how to demonstrate this for yourself. It makes more sense once you've actually seen it occur. Set the sight on a flat surface where it won't move around. Look through the sight, and with the dot centered in the scope, move the scope around until the dot is directly on something on a nearby wall, maybe a light switch, door knob, or whatever is handy. Start at maybe a distance of ten feet. Take your hands off the scope, so it doesn't move.
Now slowly move your head from side to side or up and down, and watch the dot appear to move on the wall! As you increase the distance, the effect becomes smaller. It also more or less disappears as you get farther out still. You will also notice that as long as you keep the dot near the center of the sight, it moves very little, usually not enough to make much difference.
From a practical standpoint, what does this mean? As long as gravity affects bullets in flight, you can only be sighted in at two distances, once when the bullet passes through the line of sight on the way up, and once on the way down. Shooting uphill or downhill changes at what range this occurs, but it still occurs. The higher the velocities and the lower the bullet drag, the flatter the bullet arc will be, and the less variation will occur. I would call that the parallax effect caused by sight/barrel offset. Other folks may have different terminology.
The dot moving around in the sight effect (I think I need a better word for this!) isn't as much of a problem as you might think. One of the first things new red dot shooters come up against is when they bring the gun up and they can't find the dot at all! Jerry The Geek refers to wiggling the gun around looking for the dot as the "C-More Shuffle".
With a little practice, when you bring the gun up, the red dot will be in the middle of the sight every time, just as with iron sights, where also with practice, the gun will come up with the sights aligned.
When you are sighting in a red dot sight, make sure you have the dot centered in the sight, and practice shooting with the dot in the same place every time.
Just as in everything else, there are trade-offs, and the right setup for one shooter may not be right for the next.
A large diameter red dot sight has a wide field of view through the sight, but more inaccuracy near the edges, and usually more sight/barrel offset, but the dot is easier to find.
A smaller red dot sight will have less field of view, and will usually mount closer to the barrel reducing sight/barrel offset, but the dot will be harder to find when you first bring the gun up. This is much less of an issue on a rifle than a handgun.
There are several other important considerations concerning red dot sights, including dot size, price versus quality, and so forth, and those may be topics for another post.