Your question is far too broad for a WikiAnswer. Please narrow it down to a reasonable question.
Countries will go to war if and when they believe they have a cause for war.Some countries have a policy of avoiding war, and will only fight in their own defence; while others will agressively use violence on a regular basis. Usually these agressors will try to justify their agression by claiming it to be self defence. Some other countries will fight in defense of other nations if they believe the cause is just.
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.
Generally speaking, no. A minor, by federal law, can only possess a handgun under limited circumstances, such as target shooting or hunting. If the minor is in his or her own home and uses a gun for self defense from an intruder, in most states, there will be no problem.
In general, any felony makes you inelligible to own a handgun.
It depends on what the crime is that is on your record. If it is a felony, you can't legally own a handgun anywhere in the U.S.
If you have a handgun carry permit, definitely. The handgun carry permit covers "any handgun that you legally own or posses."
By federal law you may own or possess a handgun at age 18, but you cannot purchase handgun ammo or a handgun from a dealer until age 21.
To legally purchase or own a handgun, you must be 21.
By federal law you may own or possess a handgun at age 18, but you cannot purchase handgun ammo or a handgun from a dealer until age 21.
21
18 for a shotgun/hunting rifle, and 21 for a handgun.
Self determination