There used to be something called a .256 magnum based on a necked down .357 case. It was quite powerful (on paper, anyway) but used lighter bullets than the .327 would. It was catalogued as available in Colt revolvers in 1960, which seems to be my base year for all things gun-wise. However, I don't know if any were actually manufactured. The cartridge apparently did not gain a lot of popularity.
Ruger Hawkeye, single-shot pistol.
Worth a bunch today, if you can find one.