10mm necked down to 9mm with a long heavy bullet (220 grain), with a poly tip slug would probably move pretty quick and hit hard with a nice critical defense type bullet. Fired out of a 1911 type with a compensated 5 inch barrel.
How about a 44 magnum cartridge (minus the rim) enlarged (entire shell, not just necked up) to fit a .45 acp.
It would have more power then a 44 mag, and have a bigger bullet too.
50 S&W necked down to 10mm loaded with magic dust and had the recoil of deuce deuce. The 10mm super auto mag! And it would magically work in my Colt 1911.
My personal preference...Taurus PT-840 with 158gr bullet 5.0gr of Bullseye..
460 smith and wesson in a desert eagle.
While were at it 460 S&W in an AR-15 Upper.