The HK USC Carbine in .45 - because you suck, we hate you, and 16" barrels give lower velocities in .45 than 8" barrels.
First decide if you are willing to drive to the rifle range. If you are, get a AR-15 in .223 without hesitating, or consider a Saiga for an alternative.
If you aren't, seriously consider an AR-15 in 9mm. It's relatively cheap to shoot, it would be effective if needed for defense, and you can get fairly decent 30rd mags from C-Products. It would also accept a .223 or any other caliber of AR upper if you later want another option. You could get an AR with both 9mm and .223 uppers for around the price of a single USC carbine, and it would be cheaper to shoot.
Some other pistol caliber carbines worth considering:
Beretta CX4 Storm
Uzi carbine from Vector Arms
Ruger PC9 and PC4 (latter is discontinued, but likely still in stores)
Lusa (from ptr91.com company; HK look, but based on a Portugese design that is simple blowback)
HK MP5 clones from Vector Arms (at least you're getting a more interesting mechanism than the USC)
Calico carbines (anyone want a 100rd magazine?)