The 24in/45 naval gun

I have long thought that the upper limit on naval guns would be 24in, not the usual 21in. A 24in piece would fire a shot of 6912lbs (Springsharp system where the shot is 0.5x24^3) and the piece would be 464 tons (24^3 x 45 / 1340). Springsharp won't allow for a 24in gun, even though it does all the calculations. That was how I interpreted the message: it would be feasible if Springsharp would allow the gun, but it won't.