thats exactly what your script should be accomplishing right now so we are that far. now from the experience i had when creating this system for testing one thing i noticed is that the increments in the projectiles velocity can be hard to see so for testing purposes try making your slider very large and use another binding so you can rapidly fire to see the changes. and you may want to try printing the result of the multiply on tick so you can see the speed value. the visual indication will let you know if the script is working or not and if theres further problems that need to be addressed.
the link below shows a little example video of how i setup mine and how it works in game.
