bombproof vs shellproof

bombproof is a noun but shellproof is not a noun.

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

As adjectives, bombproof and shellproof are synonyms defined as:

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

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

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

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