Estimating a 21-inch shell's explosive load

How much explosive could fit inside a 21-inch naval shell?

No consistent explosive-fill figure is established. The rough calculation uses a projectile of about 1.8 tons, but shell-type and unit corrections leave the charge estimates provisional.

The calculation is a rough extrapolation from 15-inch Greenboy ammunition and 16-inch super-heavy shells. A hypothetical 21-inch projectile is estimated at roughly 1.8 tons, or several thousand pounds, with two rounds weighing about as much as three 16-inch shells. Shell type matters, and the discussion moves between armour-piercing, high-capacity, and high-explosive ammunition. A bursting charge of at least about 80 pounds is mentioned, followed by much larger possible explosive fractions, without a consistent fill estimate being established. The later clarification says the question was being treated as a high-explosive maximum. Corrections involving pounds, kilograms, and confused land-artillery examples leave the figures provisional.