Kach,
What is an 'assault pistol'?
I ask, because assault is a behavior, not a device. I really think 'Assault rifle' is a bit of a misnomer. I'd rather it be called a 'utility rifle'
As for legal barrel length, it is 16", measured from the end of the chamber. A few states do not allow "SBR" or Short Barrel Rifles, but many do. You have to register it with the ATF and pay a $200 tax BEFORE you purchase or cut down your barrel.
If you're interested in a small, light carbine for defensive/urban situations, check out the Beretta CX4 Storm. It fires 9mm from magazines that the Beretta 92 series pistols use. You can use 10 and 15 round standard mags, or 20+ round aftermarket mags. They're reputed to be very comfortable, easy to handle, very accurate, and light recoiling. 9mm ammunition is readily available just about everywhere that carries ammunition, and it's pretty cheap. (good defensive loads cost a bit more) You'll be able to practice with it a lot.