The only "pre-existing right" is the security of a free state.
I would suggest that the security of the state was the "purpose" of the formation of the union and drafting of the constitution.
What I see as the "pre-existing right" that the founding fathers had in mind is that all members of this new democracy should be allowed to fully participate in that democracy. Of course, we have spent the last 239 years trying to figure out who "all members" are and what it means to "participate in that democracy." But hey, whoever said this would be easy?
As far as the second amendment goes, I see two main things the drafters wanted to accomplish: They wanted to assure that ALL citizens could participate in the defense of the union, not just those chosen by the government. In other words, they didn't want a situation where, for example, Protestants were forbidden to participate in the militia and only Catholics were allowed.
The second thing, while not explicitly stated, seems to me clear from the context of militias of the day and the struggles that occurred in the years after. Militias were state or locally operated units and the founding fathers recognized the dangers of a centralized government that had too much military power. So keeping military power decentralized and in the hands of local governments
**and their citizens** was important to maintaining stability and justice.
Regardless of what the founding fathers intended for the second amendment, the question is how do we address it today? What is clear is that the amendment does NOT intend to allow everyone the right to bear ANY type of arm they wish at ANY time and in ANY place. This is where we need to focus efforts to get the gun problem under control. Acknowledge the people's right to have a firearm, but limit the types of weapons allowed, the exchange of those weapons and where they can be legally carried. It's an uphill battle to try and take away the right completely, nor do I think we should do that.
HBD
Whoever calls me ignorant shares my own opinion. Sorrowfully and tacitly I recognize my ignorance, when I consider how much I lack of what my mind in its craving for knowledge is sighing for... I console myself with the consideration that this belongs to our common nature. - Francesco Petrarca
"Nothing is easier than to persuade people who want to be persuaded and already believe." - another Petrarca gem.
Ignorance is a most formidable opponent rivaled only by arrogance; but when the two join forces, one is all but invincible.