M-1 vs semiautomatic firearm

M-1 and semiautomatic firearm both are nouns.

Word	Noun	Adjective	Verb	Adverb
M-1	Yes	No	No	No
semiautomatic firearm	Yes	No	No	No

As nouns, semiautomatic firearm is a hypernym of M-1; that is, semiautomatic firearm is a word with a broader meaning than M-1:

M-1: a semiautomatic rifle
semiautomatic firearm: an autoloader that fires only one shot at each pull of the trigger

Other hypernyms of M-1 include rifle.

M-1 (noun)	semiautomatic firearm (noun)
a semiautomatic rifle	an autoloader that fires only one shot at each pull of the trigger