RE: How to tell nipple size
On most sites when you order a nipple or on the paper backing of the nipple it will say.. made to fit CVA or Tradition rifles. There are other metric sizes out there for Lyman, and for black powder revolvers. So just read carefully before you place the order.
T/C - KNIGHT - GREEN MOUNTAIN BARREL COMPANY= 1/4 X 28 #11 PERCUSSION NIPPLE STAINLESS STEEL
LYMAN = 6 X .75mm #11 percussion nipple
CVA - TRADITIONS - OTHER BPI RIFLES = 6mm X 1 percussion nipple.
Also do yourself a favor and when you replace the nipple get a stainless steel hot shot nipple. They seem to direct the flame from the cap better and do increase the ability of the rifle to fire more consistantly. Plus they last a LOT longer then the old style ones.