I'm a big FN fan but will caution on the side of Five Sevin. The civilian projectiles in the loaded ammo are completely watered down, albeit still lethal. Look at the cartridge like this, a .22 Mag on steroids with centerfire reliability and bottleneck performance.
BTW, you can buy an AR upper in this paltform that accepts the 50 round P-90 magazine. Pretty sweet. Something the wife or daughter could rock and roll easily in a pinch. http://www.botachtactical.com/pwa57x28upper.html
Reload data sources. For giggles. http://www.bing.com/search?q=5.7+x+2...ox&FORM=IE8SRC
Money is an amazing thing. I f you want a Ruger chambered in Five Sevin, I'm sure someone out there will do it for the right price. Shouldn't cost any more than the typical wild cat chamber.