Infringement is a clearer concept than many SCOTUS justices will admit to. If there is a circle drawn on a surface and I put my hand just in past the edge of the circle, the circle has been infringed. If you have a right that is not to be infringed upon, any denial of that right, including any requirement to be able to exercise that right is an infringement. Perhaps the founders shouldn't have put an example of a reason to have the 2nd by including the well-regulated militia clause, but keeping and bearing are having what you deem appropriate and using/carrying as you see fit. Even Jefferson said that a firearm should accompany us all on our walks.
I appreciate TAB's idea of the value of training. But as a requirement? So if I have a certificate that says I completed So-and-So Ninja academy firearm training, does that make me safer than my father teaching me safety all my life? I'd say ultimately that we are all responsible for the actions we make regarding gun handling and we should be able to seek out whatever training that we desire. I may learn some skills that might save my life at the $1000 handgun course, but I can be plenty safe with the basic safety rules and dad's training. I do value some training still, but am absolutely against any requirement for it, or even for the requirement for a permit.