Original intent is not a great primary approach to Constitutional interpretation. When you have a Justice of the Supreme Court seriously arguing that the Framers somehow didn't mean it when they amended the Constitution to guarantee individual rights and therefore we can, apparently, simply disregard the words they used, you're too far down the rabbit hole. Discerning original intent is very hard to discern from the historical record, and sometimes even counterproductive -- a look at how Title VII came to include women as a protected class is a good example of that.
You start with the words of the law. You understand what those words mean. If that's still not clear, then original intent is one thing you look to in order to flesh out ambiguities -- you also try to understand the policy objectives, the relationship of the government to the individual, and the goal of maximizing personal liberty.
Besides, I kind of think that Madison did mean it when he introduced and advocated for the Second Amendment. Justice Breyer's mistake is suggesting that Madison was of one mind on the issue; Madison, like everyone else, suffered from intellectual inconsistencies, blind spots in his thinking caused by his personal preferences clashing with his overarching philosophy, and was even capable of changing his mind or accepting compromises when the situation warranted it. Rare indeed is the person whose views do not evolve over time and Madison, who was first and foremost a politician, had a more supple ideology than he is widely given credit for.