kanono

Start with the collision, because it is the interesting fact about this word. English distinguishes cannon (the gun, from Latin canna, a tube) from canon (the rule, from Greek kanōn, a measuring rod) by a single letter. Esperanto, spelling both by sound, ends up with one form for two roots that were never related. Context does all the work, and in practice it does it easily: nobody has ever misread kantu kanonon as an instruction to sing artillery.

The gun sense is the everyday one. A kanono is the big-barrelled piece on wheels, the thing standing outside a museum with its touchhole plugged; the verb kanonadi is to bombard, and kanonado is the bombardment. The smaller senses follow the Greek side: a kanono is a body of authoritative rules, hence the canon of scripture and the canon of a literature, and in music it is the form where one voice chases another, which is what school choirs do with rounds.

One trap worth marking. The cathedral clergyman is kanoniko, not kanono - a different word, with the -ik- on the end. And the adjective kanona belongs to the Greek half only: kanona teksto is a canonical text, never a text made of guns.