38-55 is a great cartridge.  Original groove diameter is .379".  We keep these in stock in four diameters.  Marlins and New England Arms will use .379" or larger. Over the years commemorative rifles have been made using .375 cal barrels.  .377" is the smallest we go with this bullet. The rule of thumb; use the largest diameter bullet that will easily drop into the chamber without having to give it a little final push.   

Also, we recommend the Starline "long" brass. It is the original thickness and length.