The Act that authorized the 1936 Battle of Gettysburg 75th Anniversary Half Dollar allowed for up to 50,000 silver half dollars to be struck; all of the coins were struck by the US Mint in Philadelphia in June 1937. Including 28 coins for assay purposes, a total of 50,028 coins were struck.
After a year of slower-than-expected sales activity, the Pennsylvania State Commission (the coin's sponsor) announced in August 1938 that it was turning over sales of the coin to the American Legion - Department of Pennsylvania which promptly raised the price of the coin from $1.65 each to $2.65 each. The move was designed to spur sales by creating a sense of rarity. The ploy did not work, however, and it was soon announced that the remaining coins (>20,000) were to be returned to the Mint to be melted on December 1, 1938.
The announcement created a small stir among collectors, with a brief "sales spurt" that was enough to cause the coin's ordering deadline to be extended to the end of the year (December 31, 1938). The spurt was short-lived, however, and did not cause further extensions. Eventually, ~23,000 coins were returned to the Mint, leaving a disappointing net mintage of 26,900.
The coin is one of the more popular issues among collectors today, but, at the time of its release, faced a collecting community that was tired of the exploitative abuses of past series issues perpetrated by sponsors and dealers and, largely, just said "No!" to the Antietam.
1936 Battle of Gettysburg 75th Anniversary Half Dollar

For other of my posts about commemorative coins and medals, including more on the history of the Gettysburg half dollar, see:
Commems Collection.