Look for barely used AR-15s if you want a semi auto. Some will only shoot .223 and some will do 5.56/.223.
They are a much more heavy duty gun than the Kel-Tec IMHO, and with the collapsible stock get pretty darn small.
Keep in mind if you are in certain states you may need a "Bullet Button" and low capacity magazines. Know your local laws.
I want to get a Savage bolt action with Accu-Trigger in .223 in the future. I have one in 17HMR and it's an awesome rifle/trigger setup.