A: The shape you’re talking about is now commonly called (at least in metalworking circles) a Spiculum. And they’re not hard to make. A tapered blank of sheet metal is hammered, with a cross-peen hammer, into a v-shaped groove in a piece of wood, till you’ve curved it around to about a semicircular arc, about halfway closed. Then you close up the arc the rest of the way by hammering from the outside, working it around and the edges together, which are then soldered.

Keeping the developing form in the groove will keep it from collapsing flat while you hammer, as will watching what’s happening, and working down any areas that are bending and/or creasing more sharply than the rest. The tapered shape makes it much easier to close uniformly than a straight tube is, which tends like the dickins to flatten when you don’t want it to. Also, the taper, if you’ve made the thing from any decently thick metal, will allow you to bend the finished form to gentle curves much more easily and with less distortion than a straight sided tube would do.

Several books discuss the process, which is commonly taught as a fairly basic exercise in college metals classes. In particular, “Form Emphasis for Metalsmiths”, a by now classic work by Heikii Seppa. It was this book and author which either coined or popularized the term spiculum, I believe, as well as the term anticlasic raising, another type of complex form development.