Kel-Tec PMR-30 in .22WMR, 30-round capacity semi-auto pistol.
I've had the opportunity to shoot one, and I cannot in any way recommend it for self-defense on the basis of unreliability. I don't blame Kel-Tec so much as I blame the loading. The long case and slow powder burn of the .22 Magnum don't seem to cycle the action on an automatic very well. I had the same issues with an Automag II back in the Wang Chung days.
As a self-defense loading in general, you could do worse. It certainly dumps a significant amount of energy. That's not the whole equation, but a couple of those will do enough damage to discourage most attackers.
I second the S&W 351. It's a light gun with good sights that's easy to control.