Aletheia's

Whee, not like I haven't found some best pals in friending memes, but friending over long discussions is best friending! :) Added!
(Technicaly I'm not promising more crazy metas, but they tend to assault me once in a while and demand writing them down, indeed, so.... *g* I have a bunch of masterlists up there on the sidebar if you ever feel like digging through oldies.)

probably put 250% more thought into it than dear Ed Brubaker who basically went "IT'S COOL, DAMMIT"
Then again, I suspect the whole genre is based on careful avoiding of thinking too many thoughts of this sort, or it would never exist in the first place, so maybe they should keep it this way. :)

a simple Arc Reactor shot takes the arm right off while it bounces off Steve's shield.
Yes, that was my main (maybe only) reason to think it's not vibranium. As for the BP costume, this needs some pro's opinion, but I'd tentatively say that the arm can be steel and still react to vibranium, as long as the latter is ferromagnetic, too?

might be Vibranium (inner workings maybe?), which would reduce weight drastically, but not enough to make it as durable
Makes sense. Aluminium is too soft, tho... Titanium sounds better, especially that at least here Tony wasn't around to add gold. XD

Bucky was missing his real arm from about half his upper arm, and then you hear a bone saw, so it does seem they didn't just slap the prothesis onto a stump but purposefully removed more tissue to fully integrate it.
Yes, this was what I thought. (I should have made all my assumptions and starting points more clear, but it already was becoming tl;dr more and more... XP)