The problem with mandatory training for owning or carrying a gun is that there is zero evidence that it reduces or mitigates either gun-related accidents or crime.
Not to mention that this way of thinking:
Getting a gun license could be made as difficult as getting a license to fly an airplane, requiring dozens of hours of training.
obviously leads to the suspicion that the intent behind the training requirement is to restrict gun ownership to "the right sort of people."
There is also the issue of requiring training for the exercise of a specifically enumerated Constitutional right.