You are absolutely right about the axis of the O/U and barrel rise difference. The top barrel tends to rise more with heavier bullets and less with lighter bullets, while the bottom barrel exhibits less of this tendancy. POI does open up verticaly.

I just now tried some hot 250 gr Nosler ballistic tips, then slow 320gr Woodleigh protected point rounds in the same gun, a Merkel 204, regulated for RWS 285 gr.They shoot : bottom -right on & top 1.2" at 1 'o clock.The Noslers chrono at 2420 fps, Woodleighs at 2090 fps. The horizontal didn't change. Here is what did happen at 80 yards...regulated range. (I let the barrels cool, action open for 20 min. after each 2 shot series; ambient temp 36 degrees F):

1. the 250's bottom barrel shot 2" low, and the top barrel shot about right on...maybe 1" high.

2. the Woodleigh 320's bottom barrel shot 3" high, and the top shot 7" high.

I'll try the same test in my Tikka that is regulated for the 320 Woodleighs...using 250, 270, and several different 285-286 gr loads...some factory and some handloads. I will try RWS 285, Norma Oryx 286, and S&B 286.