Correct, the 9.3X74R was never a Black powder round, but every one I know of made before 1905 was.

One exception may be the 9.3X57 Mauser, but I have not been able to nail that one down.One German customer of mine says that cartridge came out in 1902, but most others tell me it was introduced after the 9.3X74R. I have not been able to find out myself

I have done restoration work on 2 old German guns made for strange shells which had .365 and .366 bores, obviously rifled for lead bullets and I suspect they may have originally used paper patched rounds, but I was not able to find out.

But back to the subject;

I have a Ruger. It is VERY accurate. More so than I was expecting. It has the only 9.3MM barrel I have ever seen made in the USA with a 1-10" twist. I have shot 225 grain German bullets and it shoots them very well, and it also shoots 270 and 286 grain bullets very well.

I would love to hear more about experience with the 320 grain bullets in the 1-12" twist.
I hear both good and bad reports. Some say it's good and some tell me it's barely enough.
I get correspondence from many shooters in various places and the reports differ.

Yet I have never heard a single complaint about the Ruger's not handling the 320 grain bullets, so that's why I am suggesting the 1-10 or 1-11 twist.

If G.M. were to make barrels I think it would be wise to cut them at 1-10". I can see no down side to the faster twist.