Cargo shells can be filled with practically anything, as long as it can survive the accelerations within a gunbarrel. A practical tool for long-range messaging when radio is unavailable.
Cargo shells are, as their name indicates, entirely hollow. The most popular historical use of these munitions by far was to throw propaganda leaflets at the enemy in the form of artillery shells or aircraft bombs. Oceanykans occasionally use them as fast-delivery mail when telecommunications are unavailable.