I agree with you - laws should be written in as plain, simple, and timeless language as possible. Idioms are something that I'd try to avoid if I was writing laws today. However, in this instance, the operative clause of the amendment is clear:
the right of the people to keep and bear Arms shall not be infringed.
We can also understand the meaning of the amendment by examining the intentions, motivations, and beliefs of the people who wrote it -- what they were trying to achieve by doing so. There is extensive legislative history on this subject, which makes it clear that the founders believed that individual firearm ownership was an important and necessary preexisting right. The Supreme Court ruling that I've referenced goes into this history:
> By the time of the founding, the right to have arms had become fundamental for English subjects. Blackstone, whose works, we have said, “constituted the preeminent authority on English law for the founding generation,” cited the arms provision of the Bill of Rights as one of the fundamental rights of Englishmen. His description of it cannot possibly be thought to tie it to militia or military service. It was, he said, “the natural right of resistance and self-preservation,” and “the right of having and using arms for self-preservation and defence,”. Other contemporary authorities concurred. Thus, the right secured in 1689 as a result of the Stuarts’ abuses was by the time of the founding understood to be an individual right protecting against both public and private violence.
> And, of course, what the Stuarts had tried to do to their political enemies, George III had tried to do to the colonists. In the tumultuous decades of the 1760’s and 1770’s, the Crown began to disarm the inhabitants of the most rebellious areas. That provoked polemical reactions by Americans invoking their rights as Englishmen to keep arms. A New York article of April 1769 said that “[i]t is a natural right which the people have reserved to themselves, confirmed by the Bill of Rights, to keep arms for their own defence.” They understood the right to enable individuals to defend themselves. As the most important early American edition of Blackstone’s Commentaries (by the law professor and former Antifederalist St. George Tucker) made clear in the notes to the description of the arms right, Americans understood the “right of self-preservation” as permitting a citizen to “repe[l] force by force” when “the intervention of society in his behalf, may be too late to prevent an injury.”
> There seems to us no doubt, on the basis of both text and history, that the Second Amendment conferred an individual right to keep and bear arms. Of course the right was not unlimited, just as the First Amendment’s right of free speech was not, (...) [citations omitted]
https://supreme.justia.com/cases/federal/us/554/570/#tab-opi...