I think it would be better to start with smoking several of the RMC shell cases and seeing just where the interference with the chamber is. Then compare measurements of the cases to SAAMI standards. RMC makes lathe turned cases with thicker walls than some other brands, and also custom makes cases for oversized chambers. Sizing the forward part where the case is thinner will naturally be much easier than attempting to reduce the size close to the rim.
It would also be a good idea to measure the chambers of your Spanish double to see if they are the real problem. If the chambers were reamed undersize, it might be cheaper and better to have a gunsmith run a correct size chamber reamer in them that might then permit chambering of the RMC brass cases.
No way I would buy the CH4D sizing die to just resize a dozen RMC cases. Plus you also need a press with the 1 1/4"-12 thread size for the die. It would be cheaper to buy new RMC cases made to fit your chambers.
I don't think the chambers are overly undersized, because I only have issues with less than a dozen of the cases. The old antique hulls and plastic hulls are also fine. I even gave the ones that didn't fit to a friend who also has a 10 gauge, and they didn't fit in his gun.
I think I have between 75 - 80 brass 10 gauge hulls between RMC and the antique ones. Maybe the oversized ones will end up as wind chimes after all.
Thank you everyone who replied.
