I am not sure about your numbers.
From a very quick and I do stress quick Wikipedia look.
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Antietam is sometimes cited as the bloodiest day in all of American history, but the bloodiest battle in American history was Gettysburg, but its more than 46,000 casualties occurred over three days. Antietam ranks fifth in terms of total casualties in the Civil War, falling behind Chickamauga, Chancellorsville, and Spotsylvania Court House.
For the record, the numbers are dead in one day...not casualties, you are correct about casualties at Antietam in the 50,000 man range but we are talking about dead in a single day.