The T/C barrel will most likely be .375" so expect decent accuracy with jacketed bullets. As for cast bullets, size them to fit the throat, not the bore. Oversize cast bullets do no harm and won't lead like undersized ones do. The typical .38-55 chamber reamer will make a large diameter throat. The twist rate will matter too, the .38-55 usually has a slower twist rate, but you'll have to ask. Frankly I think that unless your barrel was cut with a special custom reamer I'd get the .375WCF chambering.
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