no totally that makes sense! that's why i figured i would leave it up to you re: thread vs. headcanon. bc lbr bucky would just be sitting there, staring at weird!sam. but yes, it would be sloppy and weird.
honestly, the drama part of me says tony shot off more of the actual arm, but the logical part of me says based on bucky's reaction at the time of the injury (ie: not really exhibiting pain) it's more likely to be degradation over time.