I'm at work and have found a double rifle in this caliber for sale. I've never heard of it before. When I get home I can check in Cartridges of the World but until then, anyone have any info on it? Thanks

Mike:
Did you find an answer for this?

The .450/.400 x 3 1/4 BPE used lead bullets up to that diameter. The 'standard' bullet diameter was around .408, which was expected to obturate up to .411 groove diameter, but there is a lot of variability in bore/groove diameter in the .450/.400 Express.

There were also a couple of variants, such as the Fraser and Westley Richards cartridges, and at least one smaller rook-style .400 cartridge.

The problem is that lead bullet dimensions listed in cartridge texts rarely tell the full story with regard to actual groove diameter of the rifle they were chambered in!

Thanks Marrakai. I have found a DR that I'm going to try and buy. It was listed as a .412 caliber and I had never heard of it. I have asked for a photo of the barrel flats. That should tell me what I need to know.