how do I give a projectile higher gravity?

With your 'while projectile flying' procedure, add an 'override motion delta' function, and set the x/y/z velocities to the  projectile's current x/z velocity, and the projectile's current y velocity minus (however much downwards acceleration you want to add.) Keep in mind that this will be in addition to gravity, and you might want to have a limit on the amount of downwards momentum. (Since projectile damage is based on velocity, you probably don't want to make your knife instant death if dropped from high enough. ...Or maybe you do. I'm not one to judge.)