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Hey Otis,
My 38-55's all have .379 to 380 bore diameter, that is the SAAMI spec. so yours should be that, too. I have both new and old guns and don't find much variation in bore. Many shooters don't know how to reload for the 38-55 so when they run into issues they start altering the chamber. Don't do that...getting the 38-55 to shoot well is a journey so be prepared.
Loading dies. 375 Win dies do not work, don't wast your time. Some 35-55 dies don't work with cast bullets, either, what they do is resize the cast bullet in the seating die. The best dies that I have found and the only one that works for all my guns is the RCBS Cowboy dies.
There are two different case lengths. The Starline (2.125) and the Western and Remington (2.082). Only trial and error will tell you which will work best in your gun. The 375 Win. is 2.020 and is a different cat, avoid confusing it with the 38-55. They are two different cartridges which are not interchangeable any more than a 270 Winchester can be interchanged with a 280 Remington.
Some shooters are having guns with bad bore sent to some gunsmiths to be bored to 38-55. The gun comes back with a .375 to .376 bore. Which is a hybrid and will never shoot the 38-55 to it's max potential. Too bad.
The 265 cast bullet suggested by others, does not need a lot of velocity, I run mine in the 1400 - 1500 fps range. Very pleasant to shoot and deadly. A friend shot a large black bear in which the bullet did a complete pass thru on the bear...end to end...in the throat and out the right ham. Bang flop.
The 38-55 is extremely accurate, enjoy the journey.
