I recall reading somewhere they are made from "Beryllium Copper Alloy". AmpCo does produce such a product, but apparently the nipples are made by Treso. Since there are a couple of beryllium copper alloys produced by AmpCo., one would have to ask Treso which one it uses.
These nipples are considered the best in the industry for what they are. They are highly resistant to high temperatures, so that the orifice in the nipple( the very small constricted opening) does NOT erode away, as will happen with steel, and stainless steel nipples. Because they are made from such a tough alloy, they also resist peening of the top edge of the nipple, by repeated hammer blows.
The Uncle Mike " Hot Shot ", with the hole in the top of the nipple, was designed to use blow back gases to break the copper percussion caps free( ie., Away) from the neck of the nipple, for easy removal. The holes also vent excess gases, so you don't get hammer "blow back", which can produce a dangerous situation when the hammer is pushed back far enough to allow the spent cap to move off the nipple and travel through the air at high speed.
If you have a "weak " mainspring, and your hammer comes off the nipple when the gun is fired, first check the nipple to see of the orifice has enlarged. If so, replace the nipple. This is the usual source of the problem- not the mainspring.
I did have a foreign make gun with a mainspring that was too strong at Full cock, but was a tad weak at rest( ie., when it hit the cap on the nipple.) It would bounce back on firing from the blow back gases, and even new stainless steel nipples didn't solve that problem. About 6 months before Uncle Mikes came out with their " hot shot " nipple, I figured out that I could relieve the high pressure by drilling a 1/64" diameter hole through the cleanout screw at the end of my drum to vent the excess gases. I did so, and it did work. I filed a " hair-lip Shaped V " in the front of the skirt of my hammer, allowing gases to push out on the cap towards the muzzle, and this ended the problem of having stuck caps on the nipple.
The Uncle Mike's Solution to the problem of hammer blow back is SAFER, as you don't have to worry about people standing next to you being hit by venting gases. Its no great a problem than shooting a flintlock on a line, but you do have to warn people to stand back when you fire the gun. Any grease that is in that small hole becomes a hot projectile that burns as much as it stings.
I sold the gun before The Beryllium copper nipples were available, or before I had a chance to see and buy a " Spit-fire " nipple. At the time, Stainless steel Nipples were going for $1.00 each, while the new nipples began at $6-8.00 each depending on where you found them. I was waiting for test reports on the new nipples, before spending that much more money on a nipple.
The Spitfire Nipple is made of stainless steel that is hardened. Its available from Cain's.