bombard vs letter bomb

bombard and letter bomb both are nouns.

Word	Noun	Adjective	Verb	Adverb
bombard	Yes	No	Yes	No
letter bomb	Yes	No	Yes	No

As verbs, letter bomb is a hyponym of bombard; that is, letter bomb is a word with a more specific, narrower meaning than bombard:

bombard (noun)	letter bomb (noun)
a large shawm; the bass member of the shawm family	a thin explosive device inside an envelope or package and detonated when opened

bombard (verb)	letter bomb (verb)
direct high energy particles or radiation against	send an explosive to
address with continuously or persistently, as if with a barrage
throw bombs at or attack with bombs
cast, hurl, or throw repeatedly with some missile