The 17th century folks used the term “standing army” or “regulars” to describe a
professional army. Therefore, “a well regulated militia” only means a well
equipped militia that was organized and maintained internal discipline. It does
not imply the modern meaning of “regulated,” which means controlled or
administered by some superior entity.
What did they mean by Militia?
So in modern language:
A trained and well equipped people being necessary to the security of a free state, the right of the people to keep and bear arms shall not be infringed