metal stall bars

Can stall bars be made out of metal? Is there some advantage to them being made out of wood?

I don't see why not, if you plan on using them for strength/static skills. When it comes to practicing skills for a routine the wood is a little more pliable and the grain helps with grip.

I train on metal stall bars. Personally I don't find them as comfortable as wood, but they get the job done just fine.

That's what I made mine out of. Rigid metal conduit is pretty cheap.