automatic pistol vs side arm

automatic pistol and side arm both are nouns.

Word	Noun	Adjective	Verb	Adverb
automatic pistol	Yes	No	No	No
side arm	Yes	No	No	No

As nouns, side arm is a hypernym of automatic pistol; that is, side arm is a word with a broader meaning than automatic pistol:

automatic pistol: a pistol that will keep firing until the ammunition is gone or the trigger is released
side arm: a firearm that is held and fired with one hand

Other hypernyms of automatic pistol include automatic firearm, automatic gun, automatic weapon, handgun, pistol, shooting iron.

automatic pistol (noun)	side arm (noun)
a pistol that will keep firing until the ammunition is gone or the trigger is released	a firearm that is held and fired with one hand