firing line vs stratum

firing line and stratum both are nouns.

Word	Noun	Adjective	Verb	Adverb
firing line	Yes	No	No	No
stratum	Yes	No	No	No

As nouns, stratum is a hypernym of firing line; that is, stratum is a word with a broader meaning than firing line:

Other hypernyms of firing line include class, social class, socio-economic class.

firing line (noun)	stratum (noun)
the most advanced and responsible group in an activity	an abstract place usually conceived as having depth
the line from which soldiers deliver fire	a subpopulation divided into a stratified sampling
	people having the same social, economic, or educational status
	one of several parallel layers of material arranged one on top of another (such as a layer of tissue or cells in an organism or a layer of sedimentary rock)