Seems to me that Letters of Marque and Reprisal would not be necessary if the right was protected.
"It seems to me that there is no legal reason whatsoever"
The Founders defined a well regulated Militia. A private militia does not qualify.
“The Founders defined a well regulated Militia. A private militia does not qualify.” - RP
Post your source for this extraordinary claim - or withdraw it.
You are absolutely WRONG (as usual).
The 2nd Amendment protects (not grants) the RKBA from federal government infringement (at least the feds, but let's leave that aside for the moment). Letters of Marque and Reprisal have to do with the USE of those arms protected by the 2nd. The 2nd only addresses the "keeping" and "bearing" of arms. USING arms against a foreign power is something that only Congress can authorize, either via a declaration of war or issuing Letters of M&R.