I use stainless for my nipple guns, Roger. The shortened ones for the side by side are wearing perfectly, and the stainless nipple in my .69 is a good 10 years old and the flash hole in it is the same size as a brand new one - thousands of shots. The nipples pulled every time I clean the barrels so I can flush water into and out of the bore which effectively cleans the patent breeches. The nipples gets inspected, cleaned and returned to the rifles - in perfect order.
Ampco's are the ones that peened badly for me on the English lock. The nipples with flash holes in their sides (hotshot), reduced accuracy and velocity as well. The accuracy loss was noticable at 50 yards and considerably on the target at 100yards. I dont' rember the #'s, but do remember it was considerable - like 50% larger or maybe double. That's from 2" to 3" instead of 1" to 1 1/2". I did that testing in the 90's - could be today that might not happen - I don't need to find out as I'm happy with solid nipples.