The word "Posner" is not a magic spell. The guy is rationalizing and he's wrong. He seems to be confusing the subject matter of Article I -- a standing army and navy that Congress could establish -- and the amendment, ad hoc state militias. For all that, he doesn't even address the point that the prefatory clause is not legally, linguistically, or logically a mandatory precondition on the right. And even if you were to forsake reason and insist it was, it still is a self-explanatory private right of ownership -- so citizens can answer a muster with a weapon they knew and were familiar with. Could have taken it out "Braveheart", the Scottish teaching themselves how to fight with rocks because they were not allowed to have weapons. In the US, the ham and eggers are allowed to own their weapons and when they show up as militia, they will be "well-regulated", i.e. have their Kiffin together.