"A well-regulated Militia" in 1790 was a Militia regulated or trained to: Stand in a line shoulder to shoulder; to raise, aim, and fire as one person, and then to reload and be ready to fire again as a group. A well regulated Militia would be trained to the standards of an Army of the time, only not be an expense item for the government.
That is what the term Regulated meant. If Madison hadn't been a nerd, he would have used the more common word "Trained" or even "Practiced" and we wouldn't have this problem today.
On the other side, we might not have a Second Amendment, either.
"Nobody is safe when the Congress is in session" - Sam Clemens.