Yes, it is.
Legal implies a law. Laws are based on where you are. We get questions from many different nations here. In the US, there is no limit on the size or caliber- except that firearms larger than .50 caliber without a recognized sporting use (such as a shotgun, elephant gun, etc) must be registered as a Destructive Device, and a $200 Federal tax paid on the transfer of it to you. But if you want to own your very own cannon you can (and some folks do)
There are semiautomatic versions of the Tommy gun (Thompson sub-machine gun). They are legal to own. A fully automatic Tommy gun is legal in Texas only if you have the federal license.
No you do not have to be a gun expert to own a gun. As long as you are of legal age to own a firearm and you do not have any felony offenses on your record, you can own a gun.
.500s&w
If you mean legally, yes, in most states in the U.S. a person can own any caliber rifle that is available. I believe there are 1 or 2 states that have state laws restricting caliber.
21 for hand gun 18 for long gun
Not legally. It's illegal for a felon to own a gun in any state.
NO.
Within the laws, yes.
21
Yes, if you follow the law.
Yes. Firearms of that caliber can legally be owned in Canada.