"Hollywood" certainly gives the impression that whatever is fired from an early cannon explodes instantly upon contact with the ground or target. Spherical projectiles relied simply upon a time fuze and so in reality it was not instantaneous, except when perfectly timed to do so. Solid targets, when struck by shell, generally did cause an instant explosion by the friction caused on impact. It was not always desirable to have a shell burst upon impact, sometimes it was intended to spray shrapnel onto a target, sometimes to bury the shell into earth or structure to reduce a defensive work or building etc.
Elongated projectiles employed fuzes of 3 general functions, time, percussion & concussion - usually combining two functions so that the shell burst either at a predesignated time or on impact, whichever occurred first. However this caused problems when firing at a target screened by trees and when firing over water. Impact on foliage or glancing along water set them off. Hence there was a bewildering and almost ever changing variety of different types of fuzes for RBL & RML guns. Adrian.