Why naval guns are long-lead shipbuilding items

Why is making a large naval gun more complex than milling a rifled metal tube?

Pressure resistance, accurate rifling, straightness, barrel taper, and proof testing make a large gun a demanding manufacturing job.

Naval guns must withstand firing pressures while keeping the bore straight and the rifling accurate along its full length. A defect in the grooves affects the shell's spin and can require the work to begin again. Precision manufacture takes time, especially when many guns must be produced and then proofed. The barrel also needs the correct taper and greater thickness around the breech, together with a working loading mechanism. Initial straightness matters because firing heat will distort the barrel. Boilers, engines, and steam pipes present related pressure and manufacturing demands.