upper cannon vs rerebrace

upper cannon and rerebrace both are nouns.

Word	Noun	Adjective	Verb	Adverb
upper cannon	Yes	No	No	No
rerebrace	Yes	No	No	No

As nouns, upper cannon and rerebrace are synonyms defined as:

upper cannon and rerebrace: cannon that provides plate armor for the upper arm

upper cannon (noun)	rerebrace (noun)
cannon that provides plate armor for the upper arm	cannon that provides plate armor for the upper arm