Not usually.
It depends on what you mean. A reservist can carry a firearm while on active duty if the duty requires it. Otherwise, military personel must follow the laws of the state they are in.
It depends on what you mean. Very generally speaking, if they are on duty and their duty assignment requires them to carry a gun, yes. Otherwise, the person must abide by the laws of whichever state he or she is in at the time, just like any other person.
Mail clerk is a supplemental duty - it doesn't carry its own MOS.
It depends on what you mean. While a marine is on active duty, he can carry a gun if the duty requirements allow it. If you mean can a marine carry a gun when not on duty simply because he is a member of the armed forces, no. Military personel must follow the law of whatever state they are in when they are not on base and off duty.
Strictly speaking, a full time active duty or retired police officer can carry anywhere in the U.S. under Federal Law HR218, but there are some restrictions. The officer has to abide by the laws of the specific state he or she is in currently.
No, they are not permitted to carry on or off duty. They are required to obtain a liscense if they want to carry off duty
No
Not sure what you are asking. In relation to guns, FFL stands for Federal Firearms License, which is a license to sell handguns, and generally speaking, has nothing to do with being a police officer. That said, in the U.S. police officers can carry a handgun just about anywhere, especially if it is while discharging official duty, or while on duty. Also, under federal law HR218, police officers can carry almost anywhere in the U.S. even when they aren't in their home state, but they must abide by local restrictions.
The short answer is, because the law says so (in some/most states). The longer answer is, because someone, somewhere feels that 21 is the minimum age for being able to properly handle a handgun...the same reason you have to be 21 in most states in order to drink alcohol.
Unless you are an on-duty Federal officer, you cannot CARRY a handgun on a commercial airline. However, you CAN check an unloaded firearm as part of your checked baggage. Go to the website for the TSA and for your airline for more information.
Any law enforcement officer can carry a weapon off duty. Some departments even require officers to carry weapons off duty.
Accident injuries, no matter where they happen, reduce Army mission readiness