The Second Amendment contains the wording: "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed".
Now, which well-regulated Militia are you a part of?
"Because X is necessary, Y shall not be infringed" -does not say that a person must be a member of X in order to Y .
In the parlance of the time when this was written: 1.) every free male of fighting age was considered to be a part of the militia. 2.) "well regulated" meant "properly outfitted with necessary equipment" -aka armed with a gun and ammunition.
When the call went out to the community, it was expected for every man of fighting age to assemble with their gun and ammunition. This tradition was a simple outgrowth of the medieval standard, whereupon when the local lord called, the freemen of the estate would turn out -and bring their own weapon to fight with.
--
None of this addresses the modern concern as to whether or not we should still allow individual ownership of firearms. That is a whole other discussion.