Finding myself in need of a couple of funky BNC-to-oddity cables, I bought a 6-foot BNC-to-BNC cable from the local surplus outlet, cut it in half, and commenced attaching my oddities to the cut ends of the RG-58/U.
All went well until it came time to connect to the shield. There's a foil shield, which I didn't even think about trying to connect to, and an outer layer of silvery wires.
So, as usual, I twisted the braid-or-whatever into a pigtail of sorts, and commenced soldering. It didn't work. Check solder: 63/37 tin/lead, rosin core, works on everything else. Douse joint with rosin flux. Apply excessive heat. That wire just repels the solder.
Sure doesn't feel like aluminum wire. Maybe this is steel-shielded cable, and I need to use acid flux? Or... ah, heck. I'll improvise a crimp splice.
Update: seems some RG-58-sized, but not RG-58-spec, cable has aluminum "braid". Must be what this is, and it's some sort of stiff alloy that doesn't feel aluminous. But it has "RG-58/U" printed all along it...! Anyway, the crimp splices seem to work. For now. Wasn't there some problem with aluminum house wiring?