So I always thought a parkerized gun was hassle free so i never oiled it. For a while now ive got a big old thumb print rusted on the upper part of the mag tube and cant figure why its there. Am i wrong? Do i need to oil it more? When talking to some guys at work that served long ago they said thats weird and never had that happen on their M-16s and thought it might be a factory problem. Any thoughts? Even the heat shield is a little dark like it want to rust.