The United States has no armorial authority, and frankly, an American citizen seeking a grant of arms from another country is a bit silly and pretentious, in my view. If you want arms, adopt them.

(and yes, I'm also known as Björn the Navigator, Black Boar Herald of the Barony of Tir Ysgithr, so I do know about that armorial authority, but so does anyone to whom it applies).