How long could a 20-inch gun barrel become before barrel droop made it bend?
Short answerIt depends on available metallurgy and how heavy the barrel can be; the practical limit changes markedly between eras.
NotesThe answer rejects one timeless maximum length for a 20-inch naval gun. A heavier barrel can resist bending, but that mass imposes its own mounting, handling, and ship-design costs. Metallurgical strength also changes the feasible balance between length and stiffness. A credible answer for 1900 would therefore differ from one for 1945, making barrel length a systems tradeoff rather than a calibre-only rule.