In the parlance of the day, "well regulated" meant that it worked and worked well, or was in proper order for working.
As to the militia, this was generally held that when called up, members reported with basic arm and ammunition, camping gear (from blanket to cookpot), and knew the rudiments of military drill.
Militias whose members had all the needed gear, and knew how to use it were considered "well regulated". Those that did not were in need of better regulation. The word, as used then, and in that context did not refer to written rules the way it is commonly used today.
Still in use today, although not common, is the word "regulated" when adjusting the barrels of a double barrel rifle to hit a common point of impact at a given distance, with a given load. The gunsmith "regulates" the barrels until he gets what he wants (what works). Still a valid term, within its context.
The problem with people thinking "well regulated" in the 2nd Amendment means govt rules to be followed is simply they are using the wrong context. And the biggest problem with that is when they don't know, or will not admit that they are using the wrong context.
All else being equal (and it almost never is) bigger bullets tend to work better.