I don’t know about the rest of it, and I’m waiting to see what happens, but you are mistaken about the .380 being a tapered case. 9x19 Parabellum is tapered with .394 case head and .380 case mouth, while the .380 is .374 case head, .373 case mouth. .38 Special has .379 as the spec for the case mouth, so there will be room for the .380 case to expand in the chamber. I rather imagine that it will. I doubt the slightly smaller bullet passing through the rest of the chamber after it leaves the case is going to damage anything, or we’d already see that after the last ~100 years of shooting .38 Specials through .357 chambers.