Impossible to answer. There is no registration of gun owners in the US, so no records for the information you ask for. Best estimate is several million.
You will have to go the CDC and FBI and compile the data from their reports.
6
Gun owners aren't "registered" like cars. One estimate is that there are @ 4 million or so gun owners.
If your question is can citizens of the US own a .45 cal handgun, the answer is yes. MOST of the US does not require a permit to own a firearm, and even the areas that DO require permits do not ban the .45. In some countries private citizens may not own a handgun in a military caliber, but the US does not follow that practice.
Every state is different.
No.
M1911 .45 cal
Colt 44
Depends on where you live- you did not share what state or country you live in. In much, but not all of the US, you may possess a handgun at 18. You may not purchase a handgun from a dealer until you are 21 (Federal law applies to all states) and you may not purchase handgun ammo.
yes.
No specific information, estimated at multiple millions. The semi-auto rifle is NOT a new invention, and has been produced for a little over 110 years. There are approximately 275 million privately owned forearms of all types in the US.
No records. Most of the US does not require gun registration, so there is no central record.