The world's first SPG was composed by Sam Loyd (New York Clipper, 1895). In that problem the solver had to capture all 30 pieces and pawns, leaving only the kings on the board, in the minimal number of moves.
Loyd's solution was 17.0 moves long. (It wasn't dual-free; the first dual-free SPG was composed by Dawson, 1913.) Now, it is possible to prove that the theoretical minimum is 16.5 moves: to capture 30 men we need 15.0 moves, but the first move of each player can't be a capture, and you can't even make 2.5 moves without a third non-capturing move.
It is, actually, possible to outperform Loyd and capture 30 chessmen in a 16.5-move game.
You are invited to match your wits against old Sam and see if you can discover a 16.5-move solution.