How much discretion captains have over ammunition loads

Were warship ammunition loads standardized or chosen by captains?

They were formally standardized, but captains could often adjust the mix within defined limits to suit expected needs.

Navies prescribe standard ammunition loads rather than letting each captain invent a magazine plan. Commanders nevertheless retain some latitude to finesse the allocation within authorized limits. A modern vertical-launch system provides an example: a captain may adjust part of the missile mix without discarding the approved baseline. Standardization supports logistics and planning while bounded discretion lets a ship prepare for its expected mission.