Eighteen inches is treated as a practical upper limit for a capital-ship weapon, while 20-inch designs become increasingly unwieldy. A 20-inch/45 barrel would already be about seventy-five feet long, and more extreme calibre lengths become implausible. Weight distribution and the momentum of the moving gun create mounting and balance problems for the ship. Metallurgy must prevent barrel droop or warping, and the propellant charge must accelerate the shell through the full bore without damaging the weapon. Bigger calibre therefore demands a complete industrial and naval-engineering solution rather than a simple scale increase.
Notable Point · 1:24:13-1:25:21
Why 20-inch naval guns run into hard limits
Gun calibre is constrained by barrel length, weight distribution, metallurgy, and the propellant needed to drive a shell through the barrel.