A gun for firing a round of caseless ammunition, a round of such ammunition having a projectile and a modular propellant, the projectile and a modular propellant being disposable for firing in a gun tube, includes an actuatable valve being selectively openable to the gun tube, the valve for drawing a negative pressure in the gun tube, the negative pressure acting to pull the round into contact with a breech face such that when a firing pin is forceably released, the firing pin reliably strikes a primer of the modular propellant, the primer igniting the modular propellant, the ignited propellant forcefully discharging the projectile from the gun tube. A firing pin and valve assembly and a method of operating a gun are further included.