I'm thinking about tuning my old Vulcan (when I get time...
), I've got the delrin to button the piston, will try to get a posh piston head of some sort maybe, and have thought about putting a small radius on the transfer port using some sort of tapered tool and valve grinding paste. I've got a lathe, but as you say, working at length down the cylinder is likely to cause issues with chatter or vibration.
One day maybe...