The thicker the metal that the tube itself is made of, the lower the resistance of it (i.e. analogous to using a lower gauge wire for building your coils) so as a result from that the differences in resistance between the various metal types also becomes smaller, accordingly. What that basically means is, if the metal is thick enough all the way everywhere, then the only locations where these differences can start to grow to the point of significance as far as the vape experience is concerned is in between the contacts, not the actual tube itself. So the screw threads and all the other contacts can be seen as if the metal simply is thinner in those locations when compared to the rest of the mod, as the metal surfaces that are touching each other to provide the contact have tiny imperfections in their smoothness thereby creating tiny crevasses surrounded by areas of proper contact, essentially reducing the total surface area of the contact when crevasses cannot fully be polished out, resulting in increased resistance.
The accuracy of the machining also plays a part, as the two metal surfaces need to be pressed against each other really flat with no slanting nor wavy shape surfaces, and, polishing them till they're as perfectly smooth as smooth can be is not necessarily always going to give an improvement because screw threads may become thinned out potentially worsening the problem instead. Keeping the contacts clean and free of oxidation certainy helps, for reasons that are obvious. BUT... also please keep in mind there exist contact cleaners that contain some kind of lube or electrical grease, whereas other contact cleaners leave no residue after they completely evaporate, so, because micro-arcing occurs (typically in the narrowest spaces and moving debris that has resulted from previous micro-arcing), choose only the type of contact cleaner that does not leave a residue nor lube nor electrical grease. Micro-arcing in cohort with these so-called "protective" type layers of substances coating the surfaces creates an adverse effect because micro-arcing breaks down these substances in such a way that they speed up corrosion. Screwing the parts together when they are clean provides the necessary friction to polish, and, air gets squeezed out as they are screwed tight so oxidation from the oxygen in the air is not normally a problem if they aren't stored unscrewed for long periods of time. Just regular type maintenance every once in a while to remove dirt and blow out metal dust from micro-arcing will normally be fine. Arcing spots on the firing pin can be removed with an eraser pencil or softly pressing against 1000 grit sandpaper whilst carefully turning around its central axis back and forth. Finally, the design of the contacts plays another part. The right magnitude of total surface area of the contacts in cohort with designer ingenuity can, and does, do wonders. My copper version R6Two SBTR "Project Neptune Spear" with the Samsung 20S is living proof that it does. It hits like voltage drop of the mod is actually something below zero.