I was unaware that Magnum research makes an automatic pistol of any sort, let alone one in 357.
To make a vertical box magazine work with a rimmed cartridge, you have to slant it. So the bottom cartridge is all the way at the back of the magazine, the one on top of that is slightly forward so the rim of the second cartridge is forward of the rim of the first cartridge, then the third one is slightly forward to the second one so its rim does not interfere with the second one's rim.
The more rounds you make, the more slanted the magazine has to be. That's why 22 automatic pistols normally hold no more than 10 rounds, and normally are slanted.
The Coonan is basically a 1911, and the grip is extremely large because first a 357 is long, and then you have to have the slanted magazine which requires a slanted magazine well which takes up a lot of grip space.
Now if someone made an AR in 357 SIG, that would probably give you the power you want in a cartridge that we work through a box magazine.