Wyoming passed permitless concealed carry recently, and it looks like Montana is poised to do the same. However, there is one niggling aspect of both pieces of law that bothers me. Each restricts permitless carry to residents only.
The question that comes to mind is whether or not a state may confer a benefit on its citizens and not on visitors (i.e. those with no intention to make the state their residence). There have been plenty of cases about similar things, the first which comes to mind is Saenz v Roe (526 U.S. 489 (1999). It's not on point for two reasons. First it deals with persons who move to another state and plan to stay (establish residency) and it was about welfare benefits, not firearms. Shapiro v Thompson is another one from 1969 that forbade durational residency requirements to receive welfare benefits in Connecticut. Both cases established a line of reasoning that announced a "right to travel" (an implied, not express, right) and which said infringement (of at least some sorts) on the right to travel is a violation of the Equal Protection Clause of the 14th Amendment.
More to the point, the Court has also considered Article IV Privileges and Immunities arguments in the resident/non-resident context in Baldwin v Montana Fish and Game Commission (1978). In that case Montana was sued because it charge non-residents more for certain hunting privileges than it did to Montana's citizens. There the Court held that hunting was not a type of fundamental activity that Article IV's P&I clause was meant to protect. While it came out in the negative, the holding is illuminating because a distinguishing fact was the non-fundamental nature of hunting.
Fast-forward to Heller v DC and McDonald v Chicago and you have some new landscape to consider. The Court has held that the right to protect oneself is a fundamental and individual right. In that light, where is the basis for a state's argument that the right to self-defense evaporates upon crossing state lines, assuming the party in question is not disqualified from the possession of firearms? I can't see a very good one, unless the question is analyzed with only rational basis scrutiny. Also, the "in the home" aspect of Heller and McDonald is pretty easily done away with by the "sensitive places" discussion in Heller. Had the Court not foreseen "keeping" and "bearing" arms being a right protected beyond your front door, it would never have engaged in the "sensitive places" discussion, as brief as it was.
In the interests of a more thorough consideration, I also gave thought to a case from 1985, Supreme Court of New Hampshire v Piper, where New Hampshire sought to forbid non-residents from becoming members of the New Hampshire State Bar. The Court struck down that rule, creating a 2 prong test. If such a law is to survive, non-residents must be a "peculiar source" of the "evil" the state seeks to eliminate, and the discriminatory act must be the best way to address the problem. I can't see a bunch of non-residents rolling into Montana or Wyoming, carrying w/out permits but otherwise legal gun owners, being a "peculiar source" of any "evil" in this case.
So it is beyond the state to foreclose upon the right to carry outside the home for residents or non-residents, though the state certainly may limit the method (i.e. concealed vs. open). And that's where the biggest wrinkle comes in. Both Montana and Wyoming are open carry states. I haven't checked the specifics on their open carry limitations, but I'll wager they are far more liberal than California's unloaded-open-carry formulation. If so, loaded-open-carry may be the only saving grace for the Montana and Wyoming laws. But even then, in the Piper case, the discrimination in the method of carry may not ultimately survive absent the requisite existence of a "source" of "evil" to be dealt with.