Thanks, Tinker. Yes, this is my first double rifle and yes, I am aware of the idiosyncrasies of their proper functioning. I'm aware of the standard NFB conversion using IMR-4198 or H-4198. I am also aware of several other powders that have worked for Sherman Bell and others, including RL-7, RL-15 and Varget. My interest in comparing the case capacities is to give me another reference point on pressures besides chronographed muzzle velocity for powders other than 4198. I am eagerly awaiting the 3rd edition of Graeme Wright's book. The suggestion of making a chamber cast is a good idea. The dies on my 577/450 MH are very different from the chamber on my rifle. I wish I had a better fit on that one.