What I could really use is some practical instruction or demonstration relating this principle to marksmanship. Should I join a rifle club? And if so, would MOA be their standard of accuracy measure? Or are there any good videos online? I'm generally a learn-by-doing type so the theory has gone over my head a little.
The usual standard of accuracy is group size. Some express this as MOA. When a firearm is said to shoot "sub-MOA" all that means is it will shoot a group smaller than 1-inch at 100 yards.
Group size is, IMHO, easier to visualize. I have a mil-surp Mauser. Using mil-surp ammo I can shoot 4-inch groups at 200 yards. That can be expressed either as 4" groups, or 2 MOA.
The difference is that MOA is an angular measurement that is always the same angle, regardless of distance. Group size is meaningless if you don't know the distance. 4 inches at 200 yards isn't a great group to a world-class high power rifle competitor, but it's not horrible for an inexpensive mil-surp rifle using inexpensive mil-surp ammo. 4 inches at 500 yards would be superb. 4 inches at 100 yards would be borderline sucky, and 4 inches at 50 yards would be outright terrible.
But 2 MOA is always 2 MOA, at 50, 100, 200 or 500 yards.