When guns larger than 16 inches become practical

At what level of technology could naval guns larger than 16 inches become practical?

Most plausibly in the 1950s, or perhaps the 1940s after uninterrupted development without treaty constraints.

Japanese plans for 20-inch weapons in the late 1930s and early 1940s are presented as an aspirational end state rather than mature capability. Achieving them would require unusually favorable funding, infrastructure, and research. The 1950s offer a more credible date for technical maturity. Without treaty interruptions and with continuous heavy-gun development, the 1940s might be possible, but only after a great deal of work.