Part that travels along a ray?

I'm trying to make a part travel along a ray, I know how to make the ray, and I can make a part that goes along the entire ray like a regular raycast gun. But I'm trying to make a part that starts at the beginning of the ray, and moves along the ray till the end of the ray without appearing as a line.

local look = pilot.Character:findFirstChild("Shuttle").Cannon.CFrame.lookVector
local start = pilot.Character:findFirstChild("Shuttle").Cannon.CFrame.p
local ray = Ray.new(start,look.unit*300)
local ignore = {pilot.Character:findFirstChild("Shuttle"),pilot.Character}
local hit, position = game.Workspace:FindPartOnRayWithIgnoreList(ray,ignore)

local distance = (position - start).magnitude
local RB = Instance.new("Part", game.Workspace)
RB.Name = "Laser"
RB.Material = "SmoothPlastic"
RB.FormFactor = "Custom"
RB.TopSurface = "Smooth"
RB.BottomSurface = "Smooth"
RB.Anchored = true
RB.CanCollide = false
RB.Transparency = 0.5
RB.BrickColor = BrickColor.new("Really blue")
RB.Reflectance = 0.4
RB.Size = Vector3.new(0.2, 0.2, distance)
RB.CFrame = CFrame.new(position, start) * CFrame.new(0, 0, -distance/2)

But if I change the distance to a number in the line below, then the part just spawns at the end of the ray instead of spawning at the beginning and traveling along the ray.

RB.Size = Vector3.new(0.2, 0.2, distance)