Prof. Eugene Volokh of the UCLA Law School published a paper on the prefatory clause of the Second Amendment awhile ago. He points out that you can find other "rights" clauses in state constitutions of the 18th Century that include a prefatory clause that gives one explanation for the right. And that's all that the clause is, a justification for the right not a restriction.

At the time, "regulated" meant well trained or well drilled. Indeed, that's why we had the term "regulator clock" in the 19th Century.