punching bag vs punch bag

punching bag and punch bag both are nouns.

Word	Noun	Adjective	Verb	Adverb
punching bag	Yes	No	No	No
punch bag	Yes	No	No	No

As nouns, punching bag and punch bag are synonyms defined as:

punching bag and punch bag: an inflated ball or bag that is suspended and punched for training in boxing

Other synonyms of punching bag include punchball, punching ball.

punching bag (noun)	punch bag (noun)
an inflated ball or bag that is suspended and punched for training in boxing	an inflated ball or bag that is suspended and punched for training in boxing
a person on whom another person vents their anger