shellproof vs bombproof

shellproof is not a noun while bombproof is a noun.

Word	Noun	Adjective	Verb	Adverb
shellproof	No	Yes	No	No
bombproof	Yes	Yes	Yes	No

As adjectives, shellproof and bombproof are synonyms defined as:

shellproof and bombproof: able to resist the explosive force of bombs and shells

shellproof (noun)	bombproof (noun)
	a chamber (often underground) reinforced against bombing and provided with food and living facilities; used during air raids

shellproof (adjective)	bombproof (adjective)
able to resist the explosive force of bombs and shells	able to resist the explosive force of bombs and shells

shellproof (verb)	bombproof (verb)
	make safe against bombings