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.