How do I fix my gun "aim" view for my FPS game?

So I am making a weapons system within my game and I need to be able to "aim down" sights when the player holds the right mouse button.

Here is what I have so far:

UIS.InputBegan:Connect(function(input)
    if input.UserInputType == Enum.UserInputType.MouseButton2 then 
        held = true
        -- mouse button2 is being heldown

        if game.Players.LocalPlayer.Character:FindFirstChild(script.Parent.Name) then
            while held == true do
                if (cam.CFrame.p - cam.Focus.p).Magnitude < 0.6 then
                    --- checks if player is in first person
                    isAiming = true


                    local currentCamera = workspace.CurrentCamera
                    local startPos = script.Parent.FirstPersonViewPart
                    currentCamera.CameraType = Enum.CameraType.Follow
                    currentCamera.CameraSubject = startPos
                    currentCamera.Focus = startPos.CFrame   

                    --- sets the camera's position to the FirstPersonViewPart's position
                else
                    currentCamera.CameraSubject = game.Players.LocalPlayer.Character.Humanoid
                    -- if player let go of button2 it will return the camera to default

                end
                game:GetService("RunService").RenderStepped:Wait()
            end
        end


    end
end)

I am looking for something like this: https://www.youtube.com/watch?v=92fSHmz03Jw * I don't care if it's a smooth transition or not

But the result I am getting is that the camera rotates around the FirstPersonViewPart when the player tries to look around while aimed in.