I talked to RCBS and Freedom Arms about this a few years ago. I don't remember the details (sorry), but both gave me a clear word: you can load .45 Colt with the .454C dies, but not the othe way around. Actually, RCBS told me that due to the difference in the crimp (IIRC), to load both you needed a .454C die set and a .45 Colt die (seater/crimp die, IIRC). Sorry not to have more details, but I would recommend you talk to RCBS tech support tomorrow so you get correct info.