"Well regulated" in 18th century terms meant the same as "well trained." For musket fire to be effective, a complex, coordinated sequence of manual actions were necessary for a firing line to deliver effective volleys. A "well regulated" line would deliver sequenced volleys of shot with maximum effectiveness (i.e., killing power).
"Militia" in the 18th centry terms meant "able bodied men of the general populace." It wasn't the national guard, it did not refer to states sending trooops to the federal army -- it meant "the people."
Beyer's got some nerve citing "historians" when it is clear that he's never read any. Or at least, any competent ones.
I’d imagine Beyer has read the history. He just has an agenda to revise it to something he wants.
The idea of the “little people “ actually having firearms probably scares him.