SEC. 13. That a well-regulated militia, composed of the body of the people, trained to arms, is the proper, natural, and safe defence of a free State; that standing armies, in time of peace, should be avoided, as dangerous to liberty; and that in all cases the military should be under strict subordination to, and governed by, the civil power.
[link|http://www.libertyhaven.com/theoreticalorphilosophicalissues/history/fatherbill.html|George Mason], of Virginia, was greatly responsible for the VA and US Bills of Rights. I don't know enough about the documents to say why the text was changed, but it seems clear that the 2nd Amendment comes from a document that was concerned with state militias. Perhaps someone can find the time to dig through [link|http://www.foundingfathers.info/federalistpapers/fed46.htm|Federalist 46] and related documents to shed some light on this.
Cheers,
Scott.