rerebrace vs upper cannon

rerebrace and upper cannon both are nouns.

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

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

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

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