Well aimed shot from a pistol or rifle? Pistol, I would look at either 40cal or my fav 45acp. My person feeling is the 9 is too small, but that just me. I like big and slow when it comes to pistol ammo. A 9mm will put them down don't get me wrong but I have heard of the hopped up junkie taking 5-10 rounds before stopping. The 45acp will drop just about anyone with 1 shot. Being "gun dumb" is not a bad thing. I would suggest going to a good, reputable gun shop ask these same questions and even test fire a few before buying if that is an option. Try and find a shop that has been in business for along time and has a range where you can try different guns. Handguns are not a one size fits all proposition. There is also a lot of info on the 'net as well, but be careful what you read.
As for rifle, I don't think I would go any smaller the .223. If you want to get 8-10 shots off in a hurry a semi-auto will be the way to go. Look at the AR15 platform or AK47 for rapid fire. Provided that you don't live in an area of the country that limits magazine capacity, like the Peoples Republik of Kalifornia, or New York, the AR and AK will hold 30 rounds. 100 round mags are also available for the AK but it does make it a bit heavy. Ammo for the AR is also cheap and plentiful, the AK is also readily available. But the AK will take a larger round, 7.62mm which is equivilent to a .308. Both calibers are VERY popular.