"A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms when serving in the Militia shall not be infringed."

If we're trying to preserve the rights of States to have well-regulated militias, then the solution is far simpler.

Just make it clear that the 2A applies only to the federal government (absolutely), and it doesn't apply to the State governments. The feds can never infringe the right to bear arms, and States can adopt whatever regulations they want - since it's their call what state of armed populace serves the goals and requirements of the well-regulated militia.

If we're looking at it through the "protect the State militia power" framework, things like a federal AWB are pretty indefensible. The Federal government shouldn't be making the choice to prohibit the types of weapons that people owning (or being familiar with) would be most useful for a militia. States, however, would be perfectly free to ban them if they felt the downside to their populace was not worth the benefit to their militia's readiness.