Firearm and Six-gun

Firearm noun - A portable weapon from which a shot is discharged by gunpowder.

Six-gun and firearm are semantically related. In some cases you can use "Six-gun" instead a noun "Firearm".

Six-gun noun - A pistol with a revolving cylinder (usually having six chambers for bullets).

Firearm and six-gun are semantically related. You can use "Firearm" instead a noun phrase "Six-gun".