Fish, thank you for taking my comments well. I was concerned that I might offend you. Those are very good-looking arrows you made.
The problem with dowels comes from the way the log is cut in the sawmill. The first thing a mill does is square up the log. Logs are always tapered, so squaring it up requires that more is taken off the butt end than the tip end, so the lengthwise grain is cut at a slight angle from butt to tip. That's why most dowels have some grain runout. Different species have different amounts of taper, and that's one reason yellow poplar is better than most as dowels. The logs tend to have less taper than most species.
When I was manufacturing Superceder shafts, we used a lot of yellow poplar. We trained our log suppliers to bring us only straight, clear logs with straight bark patterns by paying a premium price. Even though the log taper wasn't much, we still cut our logs parallel to the grain, in alignment with the bark, so that when we took the last cuts from the log, the leftover center piece was tapered. As a rule of thumb, if we couldn't trace at least one growth ring for at least 3/4 of the length of the shaft, it went into the cull bin. Most of our shafts had rings that stayed in the shaft for the full length. That also helps to keep them straight.
Generally speaking, hardwood logs tend to have more taper than softwood logs, like Douglas fir and other Western US species that are popular for shafting material.