Self defense, collecting, shooting.
No.
Handguns were invented as a way to have a small portable gun (as opposed to a rifle) that was easy to keep on your person for defense.
There were single shot pistols and revolvers.
No. You do need a permit to purchase, but no registration.
You need to visit the BATFE website. They have the answer to your question.
No one source of information.
Federal law = 18 for long guns and 21 for handguns
You need to go to the CDC and FBI websites.
Check with your local police.
Yes, handguns have a serial number.
That's a very difficult question to answer because, of course, any caliber can kill. Smaller caliber handguns are fairly common in crime involving handguns because people believe large handguns are too diffucult to conceal. Most major police agencies in the U.S. use 9mm or .40 caliber handguns. It's also difficult to answer because information on caliber used in homicide and other crimes isn't readily available.
Yes, handguns were used in WWI, and WWII.