... Hawg is right on.
In this day and age, so as not to get burned, you need to:
#1) have all the nipples in your guns/pistols the same brand.
#2) Take a nipple out and take it with ya when you go to buy caps and check each tin for fit (no matter WHAT brand you prefer).
#3) If ya order them on-line, you're taking your chances plain and simple. Be prepared to throw away an entire tin that doesn't fit properly.
Black Powder: Not because it is easy, but because it is hard, because that goal will serve to organize and measure the best of our energies and skills, because that challenge is one that we are willing to accept, one we are unwilling to postpone, and one which we intend to win...