Cases for the .38-55 are longer, so need to be trimmed back to use in a .375. Even more importantly, many loads for the .375 are compressed and the thin mouth of the .38-55 may not have enough neck tension to hold the bullet in place against compressed powder. Finally, I concur with Graybeard, straight-walled cases do not have the stretching problems that necked cases do, so you should get good case llife.