Nobody seemed to know the answer to this quesstion on art of the rifle so maybe I will have luck here. How do I tell the difference between a narrow base and wide base gas tube and are they interchangeable, or is the right tube dictated by the barrel which is dictated by the receiver or... what? My garand receiver is a very low 1 million serial number SA and looks by info on the net to have been built around December of 42. Thanks