As I remember from looking into it five years ago, a liberal interpretation of the 3/5ths clause resulted in the high court’s judicial stamp of approval for slavery in the Dred Scott case. Conservatives have argued that had the constitution been rightfully and strictly interpreted as person meaning person, then the Dred Scott case would have found slavery unconstitutional. In later years, Fredrick Douglas believed the constitution, as created, was a freeing document, and you could look up his argument if you were so inclined. But as usual, Ed is incorrect in his revisionist history.
Clearly, the Constitution was never intended to allow slaves to keep and bear arms, since individuals who were free to keep and bear arms would not be slaves.
Is there any logical problem with interpreting "the people" as referring to all free persons, bearing in mind that even today not all people are free? Obviously the government wouldn't like such an interpretation, but I can't think of anyone who may be legitimately disarmed who could not also be legitimately regarded as "not free".