The way I would do drop is to set a speed (this varies of course).
Once you have set the Speed you would fire an event which tweens the bullet.
Make the script look at how many studs the bullet have travelled and then convert those studs to meters (use this link if stuck : How many studs is there in a meter)
Then, set a loop so for every 5 meters (for example), the bullet would drop by 1.5 studs.
That’s a basic roundup on the basic principles which need to apply. The rest however, is yet to be discovered by myself.