cannon vs ordnance

cannon and ordnance both are nouns.

Word	Noun	Adjective	Verb	Adverb
cannon	Yes	No	Yes	No
ordnance	Yes	No	No	No

As nouns, ordnance is a hypernym of cannon; that is, ordnance is a word with a broader meaning than cannon:

Other hypernyms of cannon include artillery, gun, heavy weapon.

cannon (noun)	ordnance (noun)
a shot in billiards in which the cue ball contacts one object ball and then the other	military supplies
lower part of the leg extending from the hock to the fetlock in hoofed mammals	large but transportable armament
a large artillery gun that is usually on wheels
heavy automatic gun fired from an airplane
(Middle Ages) a cylindrical piece of armor plate to protect the arm
heavy gun fired from a tank

cannon (verb)	ordnance (verb)
fire a cannon
make a cannon