I don't have my data handy at the moment, but that figure of 12.2 million tons seems a bit short.
The U.S. supplied many nations in addition to it's own forces.
The U.S. produced 41.4 billion (yes, billion) rounds of ammunition for rifle/pistol during the war.
So taking into consideration the stockpiles from WWI and additional ordnance produced in the interwar years, plus what was manufactured between 1940 and 1945, the total munition tonnage should be considerably more.