There are so many versions of the 38-55 out there that having a drawing or ten won't do you any good. What you need is a chamber cast from your actual barrel to see what you have. And yes, the much older black powder chamber is designed to use bullets cast from 20-1 or similar soft lead alloy at or just above bore diameter. If you try to use that chamber with smokeless powder and groove diameter bullets you will run into problems. That said, it seems silly for a manufacturer to make a modern made rifle with a chamber dimensioned like that since the majority of people buying them will be using them with smokeless. Have to wait and see what you end up with.