"Why do you need an AR15?"

[stanc]

Well Regulated?

The phrase "well-regulated" was in common use long before 1789, and remained so for a century thereafter. It referred to the property of something being in proper working order. Something that was well-regulated was calibrated correctly, functioning as expected. Establishing government oversight of the people's arms was not only not the intent in using the phrase in the 2nd amendment, it was precisely to render the government powerless to do so that the founders wrote it.

So, if something is “well regulated”, it is “regular” (a well regulated clock; regular as clockwork).

In the military context, "well regulated" meant properly drilled, disciplined, and organized.