For the most part, Hangtime is correct.
But it wasn't the bow that was the death of the heavily armored Knights, it was the gun. A bow could not pierce a piece of standard armor, except for the PERFECT shot. Hitting right at the point where it wouldn't deflect in any direction but would pierce because it has nothing else to do.
The Knights of that time were more worried about joints and their infantrymen then anything else. It was only bad luck that an arrow would bounce off the armor, but catch a weakpoint at the joint.
Second generation guns could easily penetrate ANY armor. It got to the point where every armor maker would test his armor with a pistol at extremely close range. Every piece of armor sold would have a dent in it from the test firing of a pistol. If the pistol penetrated the armor, the armor was scrapped.
Anyway, the centuries of Bowmen militia was perfectly mirrored to the american militia of riflemen. Just like the Bowmen, every single american back then knew how to use a rifle, and how to use it accurately. This gave us a byproduct that we had (somewhat) effective soldiers when needed. It's just that it wasn't (necessarily) mandated that we know how to use firearms. It just came from the lifestyle.
