Spray gun apparatus comprising:- a main body having a fluid transfer chamber and a fluid nozzle; a fluid throughbore for allowing fluid to be introduced into the apparatus; a fluid-release needle provided within the fluid transfer chamber, and operable to selectively open and close the fluid nozzle; a propellant gas throughbore for passage of a propellant gas flow into a propellant annulus proximate the fluid nozzle; a horn gas throughbore for passage of a horn gas flow into horn outlets; an inner valve member located within the main body and provided with first and second release bores (44, 46) which are alignable with the respective propellent and horn gas throughbores (14, 16) for controlling the quantity of gas flowing therethrough; a main trigger lever pivotably connected to the main body for actuating the inner valve member and a fluid flow adjustment mechanism for allowing a user to adjust the quantity of fluid sprayed from the gun for a given displacement of the main trigger lever; wherein the main trigger lever is provided with an engagement surface adapted to engage with a corresponding engagement surface of the fluid flow adjustment mechanism in order that pivoting movement of the main trigger lever results in translational movement of at least a portion (68, 74) of the or each fluid flow adjustment mechanism to thereby selectively spray fluid from the gun upon displacement of the main trigger lever; and characterised in that both the propellant gas throughbore and the horn gas throughbore are each independently connectable to an external gas source via separate gas supply fittings (18, 20). a main body having a fluid transfer chamber and a fluid nozzle; a fluid throughbore for allowing fluid to be introduced into the apparatus; a fluid-release needle provided within the fluid transfer chamber, and operable to selectively open and close the fluid nozzle; a propellant gas throughbore for passage of a propellant gas flow into a propellant annulus proximate the fluid nozzle; a horn gas throughbore for passage of a horn gas flow into horn outlets; an inner valve member located within the main body and provided with first and second release bores (44, 46) which are alignable with the respective propellent and horn gas throughbores (14, 16) for controlling the quantity of gas flowing therethrough; a main trigger lever pivotably connected to the main body for actuating the inner valve member and a fluid flow adjustment mechanism for allowing a user to adjust the quantity of fluid sprayed from the gun for a given displacement of the main trigger lever; wherein the main trigger lever is provided with an engagement surface adapted to engage with a corresponding engagement surface of the fluid flow adjustment mechanism in order that pivoting movement of the main trigger lever results in translational movement of at least a portion (68, 74) of the or each fluid flow adjustment mechanism to thereby selectively spray fluid from the gun upon displacement of the main trigger lever; and characterised in that both the propellant gas throughbore and the horn gas throughbore are each independently connectable to an external gas source via separate gas supply fittings (18, 20).