357 Rem Max case, tapered with a 223 rem die cut off to remove the shoulder. Chamber cut with a 223 reamer for 1.4" and the last .200" cut with a 338 neck and throater, ground to .361". Seating die cut the same way. Make a bushing die for neck sizing if you wanted to shoot fixed ammo.
Should be quite an accurate round for a single shot firing cast bullets, and could be used with smokeless or black. I am thinking about doing one in a stubbed barrel handi rifle just to see how it behaves, wish I had a modern made low wall, #3 Ruger, or other strong single shot action to build one up into a really nice rifle. Could be used as a "whisper" with a fast twist barrel shooting 300gr. SMKs through a can if desired.