I believe you'll be "Splitting Hairs" by slugging the bore of your gun. A 38/55 will have variations in grove diameter from one manufacturer to another. A Lyman mould for a 375 Winchester or any commercial manufactured cast bullets for the 375 Winchester will do fine, unless it's pin point accuracy you're after. Then, you'll have to be concerned about weight variations once you get the grove diameter dialed in. Load them on the light side if it's an older gun and work up slowly.