water pistol vs water gun

water pistol and water gun both are nouns.

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

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

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

Other synonyms of water pistol include squirt gun, squirter.

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