Why a 20-inch/54 naval gun never became practical

When was metallurgy mature enough for a workable 20-inch/54 naval gun?

It never reached that point in practice; continued battleship-gun development into the 1960s might have made it possible.

No date is given when metallurgy made a working 20-inch/54 naval gun practical. Large battleship-gun development stopped before reaching that point. Continued work into the 1960s is suggested as a possible path, with no claim that success would have been certain. The exceptionally long 1,080-inch barrel illustrates the scale of the engineering problem.