Why shell weight does not set a simple powder rule

Is there a rule of thumb connecting shell mass and propellant mass?

No single ratio applies because powders, charges, guns, shells, and required range differ; each firing problem has its own charge arrangement.

The answer rejects a universal shell-to-powder ratio. Naval propellants are controlled mixtures rather than one interchangeable pure substance. Propellant chemistry, charge arrangement, required range, shell weight, and the particular gun all shape the solution. Designers therefore work through a specific ballistic problem rather than applying one rule of thumb to every weapon.