It should retain zero as it zooms.
(At least that's the design intent)
That's the intent, but in actual practice, your mileage might vary. I kow that I've seen small changes in POA as the magnification changes. I have scopes on two of my .30-30s. One, an old 2.5 bushnell, on the other, a 4X Nikon. There's nothing inherently wrong with a 3X9 on a .30-30, but for my purposes that's too much scope. Still, if you like it, go for it.