Artic,
Did you check .404 Jeffery? The 404 is a 450-base cartridge without rim. The base diameter is about .544" which is a fair amount larger than the 350. The length is fine. This case would require some pretty severe swaging of the head. I believe a better case would be an H&H full length belted case, as suggested above. The belt diameter is about .532" and in front of the belt is about .511". The conversion would be a pretty simple lathe job to turn the belt to about .519", the rim to about .525" and possibly deepen the extractor groove to fit (if required). If these dimensions are found to work a special form tool can be ground to do the conversion with one operation and using appropriate stops the cases will all be the same. I suggest .375 H&H cases would be easier than 300 Mag to convert. The .511" ahead of the rim will expand upon firing. These cases are more available and would be cheaper than Bertram's.
Mike
Last edited by Der Ami; 10/13/24 12:43 PM.