When we want to apply the Second Amendment on our time and look at the word "necessary", how should we take its meaning? How much saying that something is "necessary" is open to be far removed from the current status more than in saying, for example, that something is "big" or Jim is a "doctor"? How much are you open, if you hear somebody says, for example, that a spare tire is "necessary" to a car of his, to include the meaning that, for example, he is truthfully using such expression while knowing that that car will not be used until decades from now?
But the Amendment did not stop there. It said "being necessary" instead of just "necessary". How often have you seen "being necessary" used to express an untouchable reality at a level that fits an explanation for the necessity of the militia for the security of a free state in our time? What about taking the risk of making such expression in a constitution for something that could be far fetched at this level?
Moreover, the necessity here was not expressed for itself like the example above. Instead the necessity here was expressed as the reason for the following part ("the right of the people.."). Since, by default, "being" refers to a snapshot status and not a continuous one (because that is its direct meaning), that means the beginning part does not talk about a fact that is always true. So what purpose could the attachment of the two parts around the second comma carry other than to present us with a necessity we can understand? You have to find a real necessity not a real perception of a necessity that existed back then for our time. We are responsible about the reality of necessity not the perception of the makers of the Amendment (Not that I saw a wrong perception from those people. I am just talking about theorizing). The necessity expressed in the first part of the Amendment should continuity in order for the application of the Arms clause to continue.
It is interesting to see that taking the purpose of the first part of the Amendment as merely explanatory would require that "being" was used as "always". But even if we allow ourselves to jump without proof to take "being" to be intended to imply "always" then that would even more directly lead to a touchable necessity. That is because the way with which "being" can express such meaning, if any, is an indirect one by making the listener infer that meaning from its (the listener's) knowledge that the status to which "being" refers is an unchanging one at least according to what the speaker believes. For example, one could try to make the listener infers that crossing the R river would always take time by saying : The R river being wide, its crossing takes time through dependence on the knowledge of the listener that the width of a river does not change.
While some parts here could apply to both, my focus was on the level of direct meaning of "necessary" which is beyond taking "necessary" at the "container" reference meaning level talked about previously.