How do I set the velocity to a part's look vector?

I'm creating a custom gun system but the issue I've ran into is whenever I go to shoot at a certain angle the bullet always travels in the same direction..

issue is on lines 65-69

wait()
--services
local lighting = game:GetService("Lighting")
local serverstorage = game:GetService("ServerStorage")
local workspace = game:GetService("Workspace")
-- variables
local player = game.Players.LocalPlayer
local mouse = player:GetMouse()
local character = player.Character
local torso = script.Parent.Torso
local gun = lighting.Models:FindFirstChild("AR15")
local gun2
local main
local cam = game.Workspace.CurrentCamera
local sprint = false
local walk = false
-- functions
    local function weldBetween(a, b)
    --Make a new Weld and Parent it to a.
    local weld = Instance.new("ManualWeld", a)
    weld.Part0 = a
    weld.Part1 = b
    --Get the CFrame of b relative to a.
    weld.C0 = a.CFrame:inverse() * b.CFrame
    --Return the reference to the weld so that you can change it later.
    return weld
    end

    function wepCreate()
        local a = gun:Clone()
        a.Main.CanCollide = false
        local b = gun:FindFirstChild("Main")
        a.Parent = character
        b.CFrame = CFrame.new(torso.Position + Vector3.new(-0.5, 0, -3))
        weldBetween(gun.Main, torso)
        gun2 = a
        main = gun2:FindFirstChild("Main")
    end

    function onKeyPress(inputObject, gameProcessedEvent)
    if inputObject.KeyCode == Enum.KeyCode.R then
        wepCreate()
        wait()
    end
    end

    game:GetService("UserInputService").InputBegan:connect(onKeyPress)  

    mouse.Button1Down:connect(function()
        print("Mouse pressed!")
        local ray = Ray.new(main.CFrame.p, (mouse.Hit.p - main.CFrame.p).unit * 300)
        local part, position = workspace:FindPartOnRay(ray, player.Character, false, true)

        local beam = Instance.new("Part", workspace)
        beam.BrickColor = BrickColor.new("Black")
        beam.FormFactor = "Custom"
        beam.Material = "Plastic"
        beam.Shape = "Ball"
        beam.TopSurface = "Smooth"
        beam.BottomSurface = "Smooth"
        beam.Anchored = false
        beam.Locked = true
        beam.CanCollide = false

        local distance = 50
        beam.Size = Vector3.new(1, 1, 1)
        local d = Instance.new("BodyVelocity", beam)
        d.Velocity = Vector3.new(main.lookVector)
        beam.CFrame = CFrame.new(main.Position + Vector3.new(0,0, -5))

        game:GetService("Debris"):AddItem(beam, 3)

        if part then
            local humanoid = part.Parent:FindFirstChild("Humanoid")

            if not humanoid then
                humanoid = part.Parent.Parent:FindFirstChild("Humanoid")
            end

            if humanoid then
                humanoid:TakeDamage(30)
            end
        end
    end)