Not sure where I read this, probably on this forum. As I understand it, in the days before the development of modern plastic one-piece wads with shotcup and an effective gas seal, making the chambers a bit shorter than the length of the fired shell was done deliberately as it was thought that having the end of the hull forced tightly against the inside of the forcing cone helped ballistics by preventing escape of gas around the shot column, since the card or fiber wads did not produce a very good seal.
