pump action vs action mechanism

pump action and action mechanism both are nouns.

Word	Noun	Adjective	Verb	Adverb
pump action	Yes	No	No	No
action mechanism	Yes	No	No	No

As nouns, action mechanism is a hypernym of pump action; that is, action mechanism is a word with a broader meaning than pump action:

pump action: action mechanism in a modern rifle or shotgun; a back and forward motion of a sliding lever ejects the empty shell case and cocks the firearm and loads a new round
action mechanism: the operating part that transmits power to a mechanism

Other hypernyms of pump action include action.

pump action (noun)	action mechanism (noun)
action mechanism in a modern rifle or shotgun; a back and forward motion of a sliding lever ejects the empty shell case and cocks the firearm and loads a new round	the operating part that transmits power to a mechanism