Here's a picture to illustrate the bullet arc and line of sight. Pretty exaggerated, but it's a good visual aid to see how the line of sight is straight, and a rifle barrel is angled to "lob" the bullet to a distant target. Also how the bullet crosses the line of sight twice, and "climbs". It doesn't actually climb, of course, it's just how the barrel is angled.
We all know this, but a picture is nice. It's easy to see now why a rifle zero'd at 200 yds will hit high at 100 yds., etc.
