Why a saboted 18-inch gun still depends on metallurgy

Could an 18-inch gun fire a saboted 16.5-inch projectile to gain velocity without lengthening the barrel?

Yes in principle, but only if metallurgy and sabot technology were developed far enough to make it reliable.

The suggested system uses an 18-inch bore to launch a smaller sub-calibre projectile at higher velocity. The concept is technically attractive, though its feasibility turns on metallurgy. The gun, sabot, and projectile must survive the pressure and separate predictably. The idea therefore requires a deliberate technology-development programme rather than simply substituting a different shell.