I'm not sure what you are asking. It is different in every state in the U.S. To carry a concealed weapon (one that is not visible) you must have a special permit now in just about every state. To carry unconcealed, you only need to have the gun registered in some states, but must have a carry permit in others. Normally, that covers farmers and hunters driving around with their hunting rifles in the truck, et al. For instance, it is illegal to carry a firearm into a post office or in any federal buildings or federal park or on any military installation. In other locations, some laws allow a citizen to carry a weapon openly holstered. Some allow concealed (with special permits), some not (you basically cannot get a permit). Some you must have a special permit for either/both. It gets confusing and it is up to the citizen to research and know the law before travelling. It is almost impossible to keep it all straight:

It gets murky in regards to the national park service:

CHAPTER I--NATIONAL PARK SERVICE, DEPARTMENT OF THE INTERIOR
PART 2--RESOURCE PROTECTION, PUBLIC USE AND RECREATION

Sec. 2.4 Weapons, traps and nets.

(a)(1) Except as otherwise provided in this section and parts 7 (special regulations) and 13 (Alaska regulations), the following are prohibited:
(i) Possessing a weapon, trap or net
(ii) Carrying a weapon, trap or net
(iii) Using a weapon, trap or net

(2) Weapons, traps or nets may be carried, possessed or used:
(i) At designated times and locations in park areas where:
(A) The taking of wildlife is authorized by law in accordance with Sec. 2.2 of this chapter;
(B) The taking of fish is authorized by law in accordance with Sec. 2.3 of this part.
(ii) When used for target practice at designated times and at facilities or locations designed and constructed specifically for this purpose and designated pursuant to special regulations.
(iii) Within a residential dwelling. For purposes of this subparagraph only, the term ``residential dwelling'' means a fixed housing structure which is either the principal residence of its occupants, or is occupied on a regular and recurring basis by its occupants as an alternate residence or vacation home


A part of Arizona's gun law:


Arizona

Arizona is an open carry state-- it is legal to carry a holstered or cased loaded handgun (visibly) in your car. A.R.S. 13-3102 Misconduct involving weapons; defenses; classification; definitions A. A person commits misconduct involving weapons by knowingly: 1. Carrying a deadly weapon without a permit pursuant to section 13-3112 2. Carrying a deadly weapon without a permit pursuant to section 13-3112 concealed within immediate control of any person in or on a means of transportation; F. Subsection A, paragraph 1 of this section shall not apply to a weapon or weapons carried in a belt holster which holster is wholly or partially visible, or carried in a scabbard or case designed for carrying weapons which scabbard or case is wholly or partially visible or carried in luggage. Subsection A, paragraph 2 of this section shall not apply to a weapon or weapons carried in a case, holster, scabbard, pack or luggage which is carried within a means of transportation or within a storage compartment, trunk or glove compartment of a means of transportation.


Colorado exerpt:


Colorado

18-12-204. Permit Contents - Validity - Carrying Requirements.

(3) (a) a person who may lawfully possess a handgun may carry a handgun under the following circumstances without obtaining a permit and the handgun shall not be considered concealed:
(I) the handgun is in the possession of a person who is in a private automobile or in some other private means of conveyance and who carries the handgun for a legal use, including self-defense…


In North Carolina, you have to read a book to even guess at the law:


North Carolina

TRANSPORTING WEAPONS

Given this general prohibition of carrying concealed weapons, individuals must be ever vigilant to ensure that their particular situation cannot be construed as concealing a weapon either on or about them without being properly authorized to do so with a valid North Carolina concealed handgun permit. Therefore, the person's accessibility to the weapon is of prime importance. It is for these reasons, that when transporting a weapon in a vehicle, even greater care must be exercised to ensure that the weapon is not concealed and within the ready access to an occupant of the vehicle. North Carolina law does not specifically address how to transport a weapon in an automobile. Therefore, the central question becomes: when is the weapon concealed and readily accessible to an occupant of the automobile? Obviously, a weapon would be concealed and readily accessible, and therefore in violation of our law, if it were placed in such areas of a vehicle as, under the seat of the automobile; in a bag in the back seat; in an unlocked glove compartment; or in some other manner is covered or hidden within the easy reach of an occupant of the vehicle. A previous opinion from this office was that a weapon would not be concealed if it were placed in a locked glove compartment, unless the key to the glove compartment was in the lock and the person was in close proximity and had ready access to it. A concern with this mode of transportation however is that it is quite susceptible to different interpretations, based on the various factors involved. Therefore, this may not be the most legally defensible method of transporting a weapon, and is discouraged.

While a weapon carried openly in an automobile would not be concealed, there are other problems attendant to this method of carrying a weapon. The principal drawback, of course, is in the event of a person being stopped by a law enforcement official, the officer may not readily know that person's purpose and intent for carrying a weapon. As such, it is imperative that a person immediately notify an officer of the presence of any weapon in the automobile, for the officer's and the vehicle's occupants' safety. Another obvious drawback, is that a valuable weapon may be in plain view for potential thieves to see. The prohibition to carrying concealed weapons applies not only to handguns and other weapons commonly thought of as being easily hidden, but also to "long guns" as well. Therefore, shotguns and rifles concealed behind the seat of pickup trucks, and elsewhere in other vehicles, could similarly violate our law.

As to those vehicles with no easily discernible trunk area, for example vans, the question turns on a factual determination of when the weapon is within ready and easy access to an occupant of the vehicle. If the weapon is concealed near, in close proximity to, or within the convenient control and access of an occupant, which would allow him to use the weapon quickly, then a fair probability exists that the occupant is in violation of the law. Therefore, care must be exercised by any occupant of a vehicle to ensure that the weapons are securely locked away in as remote an area as possible in relation to the passenger compartment of the vehicle. It is important to emphasize that these prohibitions apply to passengers, as well as the driver of a vehicle.
(From North Carolina Justice Academy, Firearms Laws)


>Perhaps you could clear up something for me. The guy with the permit, would he have had the gun in plain sight, e.g. on a gun belt, and walk around like that. If not, how does that sit with the offence of carrying a concealed weapon?
>Or would he have had it in, say, the glove compt. of his car and whip it out in this emergency?
>
>>And if the guy running around stabbing people had not dropped the knife? The citizen would have been forced to shoot and then he would be be under investigation and on trial with his life a mess (given our messed up system). No easy solution to situations like that; however, I think any citizen legally carrying a weapon would have done the same thing. I don't know of anyone who could sit back and watch murder (or attempted murder) and do nothing when they have a possibility of preventing it.
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>>>>http://www.msnbc.msn.com/id/17623078/
>>>It would have been better if the citizens had guns too. Here in Memphis we had a guy go crazy last summer. He was stabbing several people in the grocery store where he had just been fired. He had a good sized butcher knife and was chasing another would be victim in the parking lot. However, one of our "good old boys" happened to be armed with a carry permit and a .9 mm. He pointed his gun at the stabber and ordered him to drop it. The guy complied and nobody was shot. This scenario gets played out all over the south, but never in the great Northeast because only criminals and cops can carry guns up there....
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>>>http://www.freerepublic.com/focus/f-news/1670415/posts