Perhaps a cup bur? Be careful!

A 1/2" dowel might fit in the shank of a standard hand drill… If so, I’d use the dowel in the hand drill and would round off the other end against a belt or oscillating sander, basically.

If they’re fairly short, you could make a v block jig and drive it with a drill, and use a chisel on the side you want to round over. There’s a vid/gif I saw recently of a gentleman doing it to make the dowels themselves after knocking the corners down with a hand plane