dean, I know it seems wrong but think of this:
Rifle actions are re-barreled with barrel blanks. The blank is a cylinder that has a cylindrical chamber cut into it. It is then threaded and spun onto the action such that headspace can be set to specs. The 'smith stops turning & tightening (think of it as the equivalent of canting) the barrel when headspace is set. He does not turn the barrel so that a hypothetical part of it is 'up'. The barrel, before and after headspacing, has no up or down. It just is.
The up or down part comes when rifle sights are milled into the 'top'. A canted hold on an iron sighted rifle is bad because the sights will be off level. Sight adjustments will cause the bullet to impact in the way that MK2750's diagram shows.
It is different with a scope:
At initial sighting in, the scope can be rotated in the rings such that the reticle is level relative to the target while the shooter holds the rifle canted. Reticle adjustments will cause the POI to change in the normal predictable way: up-down and left-right. OTOH, reverting to a level hold with the same setup will cause the bullet to impact per MK's diagram.
I've never shot a rifle with a side-mount scope, but there is no doubt that the laws of optics, mechanics, flight dynamics and gravity will continue to apply.
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