How did the Wild Revolvers developer create the projectiles?

Idk what they did but what I would do is cast several short rays each starting from the end of last ray, you can also simulate bullet drop with that and put wait(x) time delay between casting rays , this will give you bullet time

They do actually use one ray which deals damage instantly once cast, they use a billboard gui frame or something similar to give the illusion that there is a bullet and that bullet has travel time. If you shoot somebody from across the map, you'll see that the damage is dealt before the bullet/frame reaches the player.