I have "turned down" nipples that were too large, or too tight a fit for my #11 caps. Different brands vary a wee bit but not enough to fix your issue. I just removed the nipple, wrapped the threads with several layers of Teflon thread tape for some protection, lightly tightened it in the chuck of my drill press, and with it turning at the slowest speed available hold a file to the cap area of the nipple and take a little off at the time, to the point that a cap slips on and stays on place while it is upside down in the drill press. Some of the ultra hard alloys may require the use of a stone in place of the file, but I never had to mess with any of them.
SRH
I've also turned down percussion nipples the same way only I just used my power drill and it worked fine. You just have to be careful not to over tighten the nipple in the chuck. When I was done I blackened the nipples with cold blue.
Steve
