water gun vs water pistol

water gun and water pistol both are nouns.

Word	Noun	Adjective	Verb	Adverb
water gun	Yes	No	No	No
water pistol	Yes	No	No	No

As nouns, water gun and water pistol are synonyms defined as:

water gun and water pistol: plaything consisting of a toy pistol that squirts water

Other synonyms of water gun include squirt gun, squirter.

water gun (noun)	water pistol (noun)
plaything consisting of a toy pistol that squirts water	plaything consisting of a toy pistol that squirts water