As a tourist from Ireland I would love to do some multi-day hiking in the Yellowstone/Bridger Teton backcountry, but I hike alone and am concerned about Grizzly bears. I carry bear spray and a big knife hoping never to have to use them, but in a worst case scenario it would be comforting to know that I have a firearm for personal protection in the event of bear attack as I realize that solo hiking in this high-density big bear country has its risks. I can probably borrow a firearm from friends for the duration of my trip if this is legal (I'd have to transport it overland from southern Utah though, which is a big diversion there and back, and brings into play Utah, and Idaho law as well as Wyoming), but obviously I do not wish to be in breach of ANY US law. Any help is greatly appreciated. Thanks!
(IANAL) Technically at a Federal level I think you cannot do this unless you meet some specific requirements. From the ATF:
An alien admitted to the United States under a nonimmigrant visa is prohibited from shipping, transporting, receiving, or possessing a firearm or ammunition unless the alien falls within one of the exceptions provided in 18 U.S.C. 922(y)(2), such as: a valid hunting license or permit, admitted for lawful hunting or sporting purposes, certain official representatives of a foreign government, or a foreign law enforcement officer of a friendly foreign government entering the United States on official law enforcement business.
From a random explanation of 18 U.S.C. 922(y)(2)
(2) Exceptions.—Subsections (d)(5)(B), (g)(5)(B), and (s)(3)(B)(v)(II) do not apply to any alien who has been lawfully admitted to the United States under a nonimmigrant visa, if that alien is—
(A) admitted to the United States for lawful hunting or sporting purposes or is in possession of a hunting license or permit lawfully issued in the United States;
(B) an official representative of a foreign government who is—
(i) accredited to the United States Government or the Government’s mission to an international organization having its headquarters in the United States; or
(ii) en route to or from another country to which that alien is accredited;
(C) an official of a foreign government or a distinguished foreign visitor who has been so designated by the Department of State; or
(D) a foreign law enforcement officer of a friendly foreign government entering the United States on official law enforcement business.
However, as per this answer to a previous Travel question about a foreigner buying ammunition leads to this ATF page:
No. A nonimmigrant alien who is lawfully admitted to the United States without a visa (e.g. Visa Waiver Program), may acquire or possess a firearm in the United States.
So it depends on how you enter the country.
But of course I would want to be carrying some pretty convincing proof of both your status, the status of your friends and the status of the weapon that you intend to borrow. At the very least you could run into Civil Forfeiture which means Law Enforcement could simply take your weapon away from you regardless of how legal it is for you to be in possession of it. (EG see Civil Asset Forfeiture: 7 Things You Should Know)
Which brings us to the individual states you mentioned:
Permit required for open carry? Hand Guns: yes, but also:
Open carry of firearm without a permit is allowed as long as the gun is at least two actions from being fired, e.g. 1) rack the slide to chamber, and 2) pull the trigger; or must carry with no bullet in the next chamber in a revolver, so have to pull the trigger twice to fire. Open carry of a loaded firearm (e.g., a live round of ammunition in the firing chamber) is allowed with a permit.
Permit required for open carry? No
Permit required for open carry? No
But of course we should review what the park itself has to say on these matters: Can I Bring a Gun to Yellowstone?
Yes, you can carry a gun in Yellowstone. But it's illegal to fire it - even in self defense. And once you exit Yellowstone, you could be in one of three states, so it's important to know the law.
(My emphasis to point out a weird fact) That site also breaks down various other facts about guns and Yellowstone.
That wasn't an official National Parks Service link (I was duped). But here is the official word:
Possession of Firearms in National Park Units
In areas administered by the National Park Service, an individual can possess a firearm if that individual is not otherwise prohibited by law from possessing the firearm and if the possession of the firearm complies with the laws of the state where the park area is located. 54 U.S.C. 104906.
Use of Firearms
Unless authorized, the use or discharge of a firearm within a park area is prohibited. 36 CFR 2.4(b) and 13.30(c). In parks where hunting is specifically mandated or authorized by federal statute, firearms may be used to hunt in accordance with NPS regulations and state laws. 36 CFR 2.2.
Visitors should not consider firearms as protection from wildlife.
The really big question is if you can safely use said weapon to protect yourself?
I would suggest that you also review questions and answers on The Great Outdoors such as