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.
Yes, it is.
A hadgun that fires a .5" projectile
fifty dollars
Fifty caliber, but it is more of a toy than a paintball gun.
100-10000 USD depending on specifics
Yes. The .50 caliber machine gun was developed for use on armored vehicles. A lighter version was designed for use in aircraft.
100-300 USD
You can find them in Greece: Tomb of Tihocan or you will get them after beating Pierre in the same level.
Yes it can
There are in excess of 12 billion firearms in the world, not counting military weapons larger than fifty caliber.
The idiom "on the wrong side of fifty" means being past the age of fifty, implying that the person is older or past their prime in some way. It can suggest that the person is no longer considered young or in their most desirable age range.
Yes. ANY caliber gun can actually kill a person, but some calibers are more efficient at doing it.