I believe it was one of the wildcats made up on the 357 Maximum case.
I've seen them done in 22, 6mm, 25, 7mm, 30 caliber.
I have a 257 maxi mag built on a small Martini action. Very accurate and gives good velocities with low recoil with light 87 grain bullets. I took a nice Mule Deer with it at 75 yards shortly after it was finished.
You've inspired me to pull it out of the safe, put a scope back on it and take it out after Coyotes again this winter. I'm getting long in the tooth for such endeavors but there are still a few easy hunts left in me.
TURF THE LIBERALS IN 2019