plastic bomb vs bomb

plastic bomb and bomb both are nouns.

Word	Noun	Adjective	Verb	Adverb
plastic bomb	Yes	No	No	No
bomb	Yes	No	Yes	No

As nouns, bomb is a hypernym of plastic bomb; that is, bomb is a word with a broader meaning than plastic bomb:

plastic bomb (noun)	bomb (noun)
a bomb made of plastic explosive	an explosive device fused to explode under specific conditions
	strong sealed vessel for measuring heat of combustion
	an event that fails badly or is totally ineffectual

plastic bomb (verb)	bomb (verb)
	throw bombs at or attack with bombs
	fail to get a passing grade