[Unclenick]

Your cheek weld is something you should be able to get into consistently because changing how your body contacts the rifle will change the point of impact. Sometimes it is significant. I've always had to dial in about a moa of right windage when I got to sitting from standing because, in the sitting position, the gun is a little less squarely on my shoulder than in standing. Hence, recoil slides the gun butt a little more toward my shoulder, causing the muzzle to move a moa to the left before the bullet gets out. My prone needs another moa of adjustment to the right with the 30's but not with the mouse gun.

Parallax is good to adjust for a couple of reasons. Having a consistent cheek weld can keep your groups as tight as they are going to be at any range, regardless of parallax. But your come-ups or windage settings may not quite match ballistic tables or other people's come-ups or windage if your parallax is right at one range and wrong at others. If your hold just happens to align your eye perfectly with the scope tube axis, then it won't be an issue. But if your eye isn't perfectly centered, an offset error occurs that grows as the range gets further from the one for which your parallax is correct. Also, as mentioned in the video, the target and reticule are clearer with correct parallax because your eye isn't having to pick one or the other as the point of best focus. Both will be within the range of your eye's ability to adjust focus, but having the virtual image of the target on a different plane from the reticule can fatigue your eye noticeably during a long match or range session by forcing it to correct the focus rather than staying relaxed.

Regarding the change in moa for bullets with range, Bart's explanation is one part. The other is that groups tend to disperse by drift introduced by slightly off-axis bullet mass or physical imperfections in the base or muzzle crown or by the bullet exiting the muzzle while the muzzle has some lateral movement, etc. The drift can be measured in inches per second away from the ideal trajectory path. As the bullet goes downrange, it is slowing down so that each successive 100 yards has a longer time of flight than the previous 100 yards did. That longer TOF gives the drift more time to move the bullet more moa away from the axis, so the group moa on the paper increases. In general, the drift is too slow to have significant drag opposing it, so it doesn't slow down appreciably and is certainly nowhere near to proportional to the drag slowing the bullet's forward travel.