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Down-range hits could put you in the slammer for a long spell.

Two points. First of all, I misspoke earlier. 55gr FMJ does not fragment much in drywall and come apart (though it will in other mediums). It does tend to lose its stability and veer off it's nominal path, and bleeds energy rapidly. It's the hollowpoints and varmint bullets that tend to fragment the most readily. In any case, you are, at best, equally as likely to have unintentional downrange hits from 9mm as you are from 5.56 / 223 Remington. The Ammo FAQ hosted on AR15.COM is is a good resource for test data to this effect

Another interesting test using drywall spaced out at reasonable room distances. The 5.56 rounds tended to veer off so much as to miss the third and fourth walls, but the one that didn't miss failed to penetrate the third wall, in contrast to the 9mm, 40 S&W, and 12 gauge slugs they shot the walls with, all of which cut right through all four walls.

Olympic Arms also has a summary of an FBI indoor penetration test of 40 S&W, 12 gauge, and .223 Remington. I'll let you read the entire report, but the money quote of the test is this: Bottom Line: In every test, with the exception of soft body armor, which none of the SMG fired rounds defeated, the .223 penetrated less on average than any of the pistol bullets.

The entire summary of the report can be read here.

The very worst case scenario of 223 vs any given handgun cartridge is that their wall penetration will be about the same. It's common, however, for .223 to penetrate less than handgun rounds because the smaller, lighter bullet bleeds energy more rapidly.

I make no recommendations, all information is offered so that you can make your own informed decision about selecting a firearm to defend yourself with.