Is a semi-automatic rifle an "arm" as the term was understood in 1789? Yes, almost certainly, in the same way an aircraft carrier is a "ship" as the term was understood in 1789. If semi-automatic rifles had existed in 1789, would the framers still have used "arms" as they did? Maybe, maybe not, but that's irrelevant. We don't care about what they thought, just what they wrote.
Likewise, is a class action "concerted action" as the term was used in 1935? Probably not. Would the drafters of the NLRA have wanted class actions to be protected had those been common in 1935? Probably yes, but that's again irrelevant.