In the DMG (p. 267), there are a number of firearms, some of which have the Reload property:
Reload. A limited number of shots can be made with a weapon that has the reload property. A character must then reload it using an action or a bonus action (the character’s choice).
Since it says, "a character must then reload," does this mean that you have to expend each of the limited number of shots before you can reload? Or, can you reload after only using some of the limited number of shots?