Elgin Gates developed a range of SuperMag cartridges based on the 357 Maximum case (OK the truth is slightly more complex and controversial, I'm not going there today).
"Paul Gauthier 12-23-2003, 08:22 AM I have one caliber in my TC Contender in that new cases must be annealed before the first loading. It is the 7mm Supermag. This caliber died a quick death after being intro'ed as the next best thing to sex. But I got a brand new barrel and redding dies real cheap so I fool around with every now and then. It's made from .357 Maximum brass necked down to 7mm with a 1/8' neck and 90* shoulder. Case life is almost non existent. Accuracy sucks to say the least. That little neck just ain't got enough surface to hold the bullet concentric. But it is LOUD, you can sure impresse your friends with muzzle flash and noise. A longer barrel might help, only got a 10". But it came and went so quickly that I don't know if they ever made any longer.If I remember correctly it was intoduced to compete with the 7mm TCU, The only difference is the supermag has more case capacity. I haven't been able to determine if that is a plus or a minus as I don't have a TCU. I think there is little doubt that either case has enough capapcity to properly utilize a 7mm bullet. Later."
There are some differences, but it is quite possible that Gates made the first 7mm SuperMag prototypes by running a 357 Maximum case through a 7mm TC/U sizing die. Except for the much shorter neck, the cartridges are dimensionally very close.
Loading data:
139 grain Hornady #2820 17.5 gr S265 Federal small rifle primers 1650 fps
