Bill,
As was discussed on the other thread you cited, double-barrelled rifles (as opposed to shotguns) were not very common in the Old West, despite their popularity in England and Europe, and with white hunters in Africa. Colt’s did produce a very limited quantity of their 1878 model double-barrelled shotgun as rifles (in calibre .45-70), for friends of Sam Colt’s son, who took over the firm after his father died. And some English and European double rifles certainly did reach the Old West. By the time double rifles chambered for metallic cartridges were available, repeaters had already reached the market, and these became wildly popular in the West. Many of May’s adventures occurred in this transitional period, where all sorts of guns were in use, from flintlocks to lever-action repeaters like the Henry rifle and its Winchester descendants.
Karl May describes the Silver Rifle and Bear Killer as muzzle-loaders, taking powder and bullets or balls at the front end of the barrels, and the two rifles owned by May are of this kind. It makes sense for the chronology involved, and many muzzle-loaders were used even after the cartridge guns came out. (In the films, for convenience, the guns are cartridge types.)
The Silver Rifle is a most unusual gun, with a stock styled in an archaic manner reminiscent to those of 17th century shoulder arms. I have never seen anything like it with provenance to the Old West, but that does not mean that a custom-made double rifle, with ornate Germanic styling could not have made it to the West, where Intschu-Tschuna somehow acquired it! There are other one-of-a-kind arms from the Old West.
I think it is exceedingly unlikely that the Silver Rifle was anything a gunsmith would simply have in stock, even in Germany. And the Bear Killer is unusually large. It resembles a mid-19th century elephant gun, except for the ornate trigger guard and belling at the muzzles of the barrels. It is a well conceived rifle.
Now, Bill, you are treading into subject matter that is very complex, but which we must ultimately introduce on this forum. It is my opinion that Karl May actually was Old Shatterhand. The precise meaning of this is another matter. I think it is possible for individuals to create their own reality, and I do not consider this process in any way a form of mental illness. Yes, Karl May suffered from emotional problems in his youth (quite understandably, for a sensitive genius born into desperate poverty), well-detailed and explained in your excellent book Karl May: A Medical Casebook. But by the time he was a successful author, all of this was behind him. His work, and his indefatigable work schedule through the years, evinces a man of rock-solid mental stability, as you pointed out in the book.
The question is really: did Karl May actually travel to America, become Old Shatterhand, and meet a great, young Indian chief—or is this part of an alternate reality, though not necessarily less real than ours? The bona fide Karl May researchers may laugh (and I hold that kind of laugh in contempt), but this is a legitimate question. There is so much accurate detail in May’s work that it is hard to imagine research conducted from Germany under the conditions of the Victorian era could have provided all of it. On the other hand, there are chronological problems. While Karl May could have travelled to America, it does not seem that there was sufficient time for all of the adventures (though of course, May could have combined real and fictitious events in his stories).
I think that we will ultimately have to embark on discussion of this topic on a separate thread. I hope that forum readers are not somehow put off by it. {

Are you out there, readers? Why not make yourselves heard here?

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To return to the subject of the pronunciation of “Winnetou”: Karl May did phoneticise many Indian names in the German manner, as in the case of Intschu-Tschuna and Nscho-Tschi, Winnetou’s father and sister. And as I stated at the outset, I always assumed that “Winnetou” was properly pronounced “Vinnetou” (as phoneticised in English). But the name Winnetou is so special (as is the character), that perhaps May reproduced an English phoneticisation. This is supported by other Indian names with the Winne- prefix. I am beginning to suspect that “Winnetou” might properly commence with the English W-sound........