Initially it meant EVERYTHING. Only members of a government-controlled "Well-Regulated Militia" could own weapons, which would obviously include M16s.
Now that the Supreme Court has explained that the right includes weapons for self-defense, the anti-gunners would now state that the Militia clause means NOTHING. Only the government can possess M16s and the citizenry is limited to weapons that are only suited to self-defense and no other purpose.
We can only hope that soon the anti-gunners will become dizzy and pass out from exhaustion so that we get a break from their illogical nonsense.
I don’t think the term “well-regulated” meant government controlled. Rather, a well-regulated militia is something beyond a ragtag assemblage of individuals but instead one that has enough experience and skill that it can be mustered into a combat unit of whatever size (squad, platoon or company) to ensure the security of a free state.
In the 1970s, anti-gun script writers were claiming the Miller Decision by the SCOTUS proved RIFLES were protected under the 2nd Amendment but handguns were not.
Lots of TV shows at that time parroted that thought.
Now, suddenly rifles are BAD, but we knew it was coming.