Well Regulated
In the language of the day, when those words were written, well regulated meant "trained and practiced, skilled."
The militia is, of course, every adult. In those days, it was every adult male, but today I would say, simply, every adult.
So, since the militia is every adult, and you want a militia that's skilled -- trained and practiced -- it only makes sense that the individual (of which the militia is made) would be armed.
How do you train and become skilled without arms of your own? "Hey, Bob, can I borrow your rifle? I gotta go get trained."
Let us not, however, get distracted by the "militia clause" of the 2nd Amendment. It's not the part that drives the enumerated right: "the right of the people to keep and bear arms shall not be infringed."
People. People is individuals. The people is everyone. Every individual.
Just so we're clear on that.