As 40 said,
The hammer face should never contact the nipple. Contact Rowdy Yates for the cure / prevention.
That's it in a nutshell. If the hammer face contacts the nipple, you'll get a mark or indent. With ANY brand of nips, save the accidental "butter-soft" one that escapes the factory before the hardening treatment.
If the length (nipple/cone) or gap is adjusted properly, only the cap will touch
both pieces - nip & hammer. I've heard figures from 0.005" to 0.01" being the right gap, altho I think any gap will do as long as the thickness of the cap metal plus the priming compound is greater than the gap.