The preamble states that the right is protected because a well regulated militia is necessary to the security of a free state.
Interesting to note that the preamble does not state that the right is protected because a well armed citizenry is necessary to the security of a free state.
That is EXACTLY WHAT IT SAYS in conjunction with the whole sentence.
Something is "necessary to the security of a free state".
What is that something?
That something is "a well-regulated militia".
What is a militia?
The militia is "civilians primarily, soldiers on occasion".
How is a militia "well-regulated"?
Among other things, it is "well armed".
How can we most easily and thoroughly achieve a "well-regulated militia", which is no less than a "well armed citizenry"?
We can efficiently achieve a well armed citizenry by protecting "the right of the people to keep and bear arms".
Who are "the people"?
The people are the aggregate of individual citizens.
Who is to "keep ... arms"?
Individual citizens keep arms - their own, at their own cost, under their own control.
Who is to "bear arms"?
Individual citizens bear arms - under the organization of Congress when called up, under their own responsible discretion when not.
Ergo, the preamble (in conjunction with that which it is a preamble to) states that the right is protected because (along with the fact that it is a natural right) a well armed citizenry is necessary to the security of a free state.
They didn't have to. That's what "the People" is for. Not the state government, not the federal government, but the People. They didn't HAVE to enumerate each person or refer to them as "the Citizenry" (good God Almighty).