I think the varying POI can be the result of a number of variables, most of which I'm not competent to explain. As you mention, harmonics constitute a variable which will change with velocity. The rotational speed of the slug will change with its velocity, and can affect its flight. Barrels are not usually straight. Most of them are close enough that accuracy doesn't suffer, but not perfect. So when the projectile exits at an angle, its rotation will cause something other than a straight line of flight, maybe a big lazy spiral, I don't know. After years of developing loads in CF benchrest, I'm not surprised at all at your observation of POI variations with velocity changes. After messing with this stuff over 50 years, I've learned a lot, and the most important thing....I really don't know much. Here is what I know: every rifle and barrel is different, and load development (tuning in air rifles), must be done on an individual basis. Don't have preconceived notions, learn from the rifle, because you'll never figure it out.
