I have used amco (Tresso) nipples. It probably doesn't hurt anything but they corrode very quickly, almost like copper or brass. They have a smaller flash hole, as does Blomquists.'
You may be interested in a conversation that I had with Ron Blomquist while I was looking into a better nipple. I posed several questions to him. I'm giving an excerpt of some of his answers below:
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My nipples have a straight flash channel. In the 15th century a man named Bernoulli proved that a straight hole is the same as a tapered one, it all depends on the smallest diameter of the hole. What you do in front or behind that hole does not affect the flow of the gas, smart guy, drank a lot. When you see a taper, or a hole in the side of the cone, it is designed to sell the nipple. The last thing I would want is a hole bleeding gas away from the breech... So having a taper doesn't really matter, the ignition source is the same and once it goes you cannot add to it. I make my nipples with .028 holes on #11 nipples. I believe that Treso uses the same size holes. I also try and balance length of a flash hole with an eye toward wear and how far away it is form the source of ignition. In other words I could make the flash hole really long and they would last forever but it would also be more difficult for ignition to reach the breech. The shortest path with decent wear, about 200 shots is what I go for.
Regarding smaller flash holes:
I have rarely seen a smaller flash hole that caused a rifle to be less accurate against a larger one. In that regards I have a T.C. Renegade .54 that I use a musket cap on with a small flash hole that out shoots a #11 by half. In that case you tell me? Your guess is as good as mine but your inclination to go with what works is the best one, try stuff, find out, move on, that is the joy of black powder.
Regarding a mainspring that was too strong, nipples were being cut:
As I said before some small amounts of grinding on side of spring will lesson weight of spring, the first thing guys do to revolvers, to improve accuracy. It doesn't need to slam nipple just break cap. If you reduce spring go slowly as it can change quickly after it seems like nothing is happening, I know this , guess how I found out? If you can get it to stop cutting the caps, I would quit. The other issue I have found with [some rifles] that the shroud on hammer is contacting barrel before face of hammer is hitting nipple which makes spring seem under powered, you might check that. Also nipples can vary in length just enough to make you mental, another, no sh_ _, moment. Now to recap: Treso makes an excellent nipple, I don't like their softness, excellent wear on flash hole though. CVA and Spitfires work mostly gimmick...