Set a variable for the fire rate. Set another variable current fire time. On tick check whether game time seconds is greater than CurrentFireTime. If it is, fire the gun and increase CurrentFireTime by FireRate seconds. Then the next time it fires will be after than much time has passed. Also when it fires decrease FireRate (maybe we should call it RefireDelay) by a small amount. And it repeats this process as long as fire button is held, gking faster and faster until FireRate is getting Clamped to a minimum value (so it doest reach firing once very frame unless you want it to) remember to multiply by deltatime.