I don't see how the federal government setting explicit standards for arms and equipment suitable for use in the event of their calling out the militia would help, but I'd not have a huge issue with it.

How about a subsidy? Even something as simple as a tax credit for the purchase of arms, equipment, and ammunition with which to train would probably be broadly popular.

... I suspect this isn't what you meant when you wrote your comment - but it is what the people who wrote and ratified the BoR understood the text of the Second Amendment to mean.

Thorough and enforced training requirements for gun ownership I think would shift a lot of opinions from the somewhat opposed to the accepting column.

This is only a tenable position to hold if you _also_ accept educational requirements to exercise other rights. E.g., the right to vote.

The Supreme Court has held that such requirements to exercise a fundamental right are inherently unconstitutional and discriminatory.

Because rights are held to the standard of scrutiny, the only way to diminish the protections afforded by the second amendment is to lower the standard of scrutiny, which opens the door to two scenarios: either the 2nd amendment is deemed unpopular enough of a right that it warrants diminished protections, which yields the prospect of any right becoming unpopular and protections similarly diminished, or none of the enumerated rights deserve the robust protections that the second amendment provides, which imperils protections on rights we may care more about, like voting (which is not even constitutionally enumerated,) speech, etc.