Hi Unclenick, Thanks for the advice in your attachment (Crown Ball Lapping2.PDF). I shall be re-crowning my Walther GPS-C barrel using it this weekend.
However, although this post is old, for the sake of posterity I think I should mention that the formula for choosing the lapping ball size is incorrect. The diameter (phi) should be: D/sin"theta", not 2D/sin"theta".
Thanks again for the info.