Don's point about the .29" hickory rod is a good one. My pipes are 5 1/6" and will be loose with a smaller rod, so I'm using a 5/16" hickory. There is hickory, and there is hickory. Gotta be straight grained. I've a couple I'm 'saving' for hunting.
I find the .32 to load so easily, that I don't have to choke up on the rod for it as much as my larger bores using 3/8". I have been using a steel 5/16", but am concerned about bore damage - some. The 5/16" steel rod is a snug enough fit, as to centre very well and not scrub the muzzle, in any event. I do like the weight of the 38" long steel rod in the gun, but might go with a 9/32" if I could find one and put some sort of tightener in the hole or on the rod, itself - just a ring of brass or something to centre it and take up the slack in the rod hole.