You may only regulate who CANNOT own a firearm, not who CAN. You may not impose training requirements or testing on the right to keep and bear arms, it would be akin to imposing educational requirements and testing on the right to vote. If the government wishes to ignore its obligation to ensure that ALL citizens (the militia) are well trained in the use of arms, that doesn't then allow it to impose training on SOME as a condition of accessing one, or any, of their unalienable rights endowed by Nature and Nature's God.
If you want a well trained militia, then the federal government needs to step up to the plate and require that EVERY citizen and legal resident capable of bearing arms undergoes that training . . . essentially reinstitute the draft and require that every person graduating from or dropping out of secondary schools undergo a specified regime of training. Truman, a progressive's progressive if there ever was one, proposed just such a program during his presidency.