"Today the US has a rather large, well equipped standing Army, Navy, Air Force, Coast Guard guard, etc. to look after the security of their “free state”, so you would think that the second amendment would be redundant."
Actually not so. The militia is comprised of much more than members of the U.S. military.  The Militia Act of 1903  and 10 U.S. Code § 246 describes the militia to be "able-bodied males at least 17 years of age and under 45 years of age" separated into two groups, organized and unorganized, the latter comprised those without military affiliations.