I made a projectile script and it works just fine but it needs some upgrades like calculating range using speed and gravity or range with speed and gravity just to make everting more clear than now script is using rigid body to add force to the projectile
finally sorry for bad english.
Im not sure what you are trying to achieve. Do you want the projectile to have a constant speed? keep accelerating? slow down due to drag? Do you want the projectile to slowly drop due to gravity like a bullet would shot over long distances? theres alot of info needed about what youre trying to do before anyone can really help you achieve it.