I recently traded for a Winchester Legendary Frontiersman Rifle in .38-55. The bore slugs at .379 and I ordered some poly/moly coated .380 diameter 255 grain boolits from Bear Creek Supply. They all measure .380 and are within +/- 1.5 grains of stated weight. The rifle came with a new set of RCBS dies, but the expander plug is for .375 bullets.
I have read that the expander die in the Cowboy Die Set is larger to accommodate larger diameter cast boolits. I found several places on line where the die is sold for about $28 and all have a 4-6 week waiting period. So, three days ago I sent an email to RCBS customer service to see if they have the die in stock and what their price would be. Today I got an email telling me that they will send me the larger expander plug that is a screw in replacement for the one in the original set, and at no cost to me. How's that for customer service? GF