Is that the one that attaches flat on the end of the muzzle with a magnet?

If you are off bore center by some amount, the dot will also be off by the same amount at the target. At range distance, you just don't notice it.

A bigger error will occur if the muzzle crown and/or bore sighter surface is not square. This is easily checked by rotating the bore sighter to several different positions. The dot should not move within the scope view.

(edited to add)

For example, if the bore sighter is shifted 1/8" off center at the muzzle, the entire length of the beam will also shift by 1/8" all the way out to the target and the dot will only shift by 1/8". But if the muzzle surface is off by just 0.1 degree, the dot will be off bore axis by about 3 inches at 50 yds. Just an example for the difference. 0.1 degree would be unusably high.

You can always find the true beam axis of a bore sighter by observing the dot position with the bore sighter at various positions of rotation. It will either be perfect with no movement of the dot, or the dot will trace out a circle. The center of the circle is the true beam axis.

Whether the beam axis aligns with the bore axis depends on the accuracy of the chamber or muzzle face for the chamber and muzzle face mounted bore sighters respectively. A bore mounted bore sighter will always be aligned of course.

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My experience over the years has shown me that generally speaking bore sighting is over rated. It is better than nothing, But I usually do not waste my time with it.

 

I have used a Sightmark boresight for a number of years. At dusk, I can usually find the red dot at about 40 yds. I set the scope based on the ballistic of the load, and I’m usually within a couple three inches at. 100 yds. I’m grousing because I expected more from Real Avid.

 

I've had pretty good luck just eyeballing around the barrel (turning the pistol) and observing the fit for variances. By this I mean does the sighter hang a bit lower or higher in any one position around the barrel? Adjust accordingly.