This is one case where the originalists like Scalia have a very good point. When we consider what that clause means we need to consider what those words meant and used when the constitution was written.

At the time the third meaning was the common usage. And this was how the framers envisioned the military working. Rather then keeping a large standing force, there would be organized groups of armed civilians that could be called up when needed.*

Unlike Scalia I don't believe those things are written in stone however. Things do change, and the key one where was cartridge cases for ammunition. This made pistols much more practical. It made it possible to load a weapon and leave it sitting around and expect it to work. This greatly changes the balance of the risk/reward equation for weapons. And does so in a way that requires more regulation then the founders probably had in mind.

One of the reasons they wrote the constitution in general broad strokes is because they wanted a document that could flex with the situation without requiring alteration for every little change.

Jay

*And part of the reason they wanted to do that was to avoid stationing a large army outside the US. They knew that if the only force available was call-up guard, it would be impossible for the US to built an empire the way the UK and France had done. In that regards history has proven them correct. It wasn't until we abandoned the idea of using the guard as our primary force that a large military build up outside the US became possible. Of course it can also be argued that the second world war made it necessary, but that is another argument.