In general, no. A person that has been adjudged to be metally incompetent may not possess a firearm anywhere in the US. This is governed under Federal law. If a court has determined that the person is no longer mentally incompetent, you may petition for restoration of firearm rights. You need to consult a NH attorney on this.
If you were ever adjucated as being mentally defective, you can't own one, period. If the court required you to be committed, you can typically count yourself out. If you entered into it voluntarily, it may be a different case. You would need a lawyer, privy to the details of your specific case, for a more detailed and accurate answer to this.
This prohibition does not apply, however, if the court of adjudication issues, upon after having been found not guilty of a crime due to mental disease or defect. Who has been hospitalized as an inpatient in any mental hospital, no person shall own, possess or control any firearm or ammunition.
One of the groups prohibited from owning a gun is anyone "adjudicated as mental defectives or incompetents or those committed to any mental institution." I don't know anything about psychiatry, but I'm sure that falls into that category, so I'm going to say, no, not legally in the U.S.
After a felony conviction, you may never own, purchase, possess, or be granted access to firearms ever again.
That will depend on the weapon, and the law of the place where you are. In most of the US, the only requirement to own a firearm is that you be an adult, of legal age, not a criminal, illegal alien, a mental subject or drug addict. In MOST states there is no permit to own a firearm. In other countries, other laws.
This ALL depends on what state you live in, and what crime you committed. In most states, you must have a clean record to possess a firearm with no felonies, so if it is a misdemeanor, it takes the time they tell you to have it expunged from your record. If it is a felony, you can NEVER own a gun legally.
I'm assuming you mean "Can a sex offender own a firearm?" If so, the answer is no. Felons cannot own firearms.
yes
No
If you entered rehab voluntarily, yes. If committed to rehab, probably not. You may want to check 18 U.S.C. 922(g) for a list of prohibited categories. The language of the law refers to "unlawful user of or addicted to any controlled substance" and "been adjudicated a mental defective or who has been committed to a mental institution" as being prohibited from acquiring or possessing firearms. Going to "rehab" could mean many things. The law doesn't mention anything about "rehab". You should try and clarify your question. Is your rehab considered a "mental institution"? Who sent you there? There are a number of questions that should be answered. This question is very difficult to answer without getting many more details from you.
Private institution z an institution own or control by one person
You can pawn a firearm which you own.. if you pawn a firearm which you do not own, the legitimate owner may report it stolen, and you will be prosecuted.