I've done it with arrows, with a hand plane and sandpaper. A light touch is called for. I used a "thumb plane" that was easy to maneuver.
I can't see any advantage for a tapered ramrod, and not much for a tapered arrow, since the bigger the better (ramrods, that is), IMO, so long as it fits down the bore.
You can make a tapered arrow or a ramrod shaper by simply attaching a fence at an angle to your sander, I think, but it's a lot of work for one or two ramrods. So long as you use a pusher stick. You can taper on both ends from the middle or one end. Would take some experimentation, at least for me!