I'm more prepared to accept linguistic and social explanations for both materials, personally. Adamantium in contemporary langauge is more or less synonymous with "super-hard-mega-metal", even though it is indeed derived from Latin. Same goes for Plasteel - I think we're all in agreement that whoever invented the metal had no intention of creating a name that alluded to any industrial process in particular - thus I'm prepared to let the name go as a 40K linguisitic anomaly.
The social development and regression of a people is routinely overlooked in science fiction and fantasy alike - mostly, I suppose, because people would rather believe that the spear creates the society rather than the other way around - and in this case is probably more relevant to our discussion than anything else, as Dosdamt pointed out. Philosophically speaking, we can only guess at how such weapons as the Bolter, Flamer etc might work, given that the definition of knowledge as "justified true belief" forces one to accept that knowledge today might not be knowlegde tomorrow.
Blimey, that was a mouthful
Tullio
