letter bomb vs bombard

letter bomb and bombard both are nouns.

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

As verbs, bombard is a hypernym of letter bomb; that is, bombard is a word with a broader meaning than letter bomb:

Other hypernyms of letter bomb include bomb.

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

letter bomb (verb)	bombard (verb)
send an explosive to	direct high energy particles or radiation against
	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