RE: Line of Sight - Line of Bore Qn.
You're right, there's nothing built into a scope to create the angle. The angle needs to be adjustable to allow for different applications.
I disagree that a bullet can cross the line of sight only once. If it crosses once, it must cross again further downrange, assuming it doesn't hit anything of course.
True, bullets don't "rise", they only fall away from the line of the bore. That is the trajectory. But, in order for it to be possible for the bullet to hit the target, it must be fired at an upward angle in reltion to the liine of site. The line of the bore and the line of sight intersect at some distance downrange, say around 25-35 yds. That's why most people take a few shots at 25 yds when a new scope is installed. If you hit dead on at 25 yds, you're on the paper at 100.
Changing the heightof the scope in relation to the bore, say with low or high scope mounts, does change the angle that must exist between the line of bore and the line of sight, but not much.