An interesting development... While debating this, someone pointed out that such training requirements might be included in the first bit of the 2nd amendment;
"A well regulated Militia,...".
To me, this could bring a slew of problems as to what is defined/considered 'regulated'. But it is an argument I cannot just dismiss/ignore nonetheless.
Someone help my brain here, it's beginning to hurt a bit.
The training requirement that "might" be prescribed in the preamble of the Second Amendment would be a mandate to the States to provide training to all its firearm owning citizens in marksmanship, weaponry, discipline and tactics as its responsibility to maintain a militia and properly defend the State against a tyrannical central government.
Unfortunately, most of the States abdicated this responsibility to the Feds when they agreed to merge their militias into the National Guard.