There were single shot pistols and revolvers.
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.
Self defense, collecting, shooting.
No one source of information.
People
yes
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.
People ate baked goose, plum puddings, and cakes in the 1860's. They also ate minced meat pie and apple pies.
Yes, handguns were used in WWI, and WWII.
number of enslaved people
Yes
The estimated world population in 1860 was around 1.2 billion people.