Puerto Rico has blatantly unconstitutional gun laws. It has a minimum wage under the federal minimum wage. The US congress requires all applicant territories for statehood have constitutions compliant with the US constitution (a problem for North Dakhota until 1997 funny enough), have laws requiring child support on the books and a whole host of other laws. Puerto Rico is compliant on some fronts and not on others.
The only thing that the U.S. constitution has to say about gun laws is "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed". Which means Puerto Rico's gun laws are not in line with the current interpretation of this cryptic sentence prevalent in the 50 current states (as in more strict - what a scandal!), but saying they are unconstitutional is a bit of a stretch IMHO.
It IS unconstitutional, because the SCOTUS says so. Whatever their interpretation is, is the law and defines what's "constitutional". Yes, even when the Justices are a bunch of partisan hacks and their interpretation is BS. You may not like it, but that's the way the law works.
Funnily enough, the second amendment isn't that cryptic. It says that, in order for the states to remain free, they need their own militia made up of the people, and that militia needs arms, thus the federal government shall make no laws inhibiting their ability to acquire said arms.
Ironically, the laws we have on the books are technically unconstitutional.
Just a nitpick, it doesn't say the federal government shall make no laws, it basically says nobody will. Precedent after the fact limited the scope of the bill of rights to the federal government until the passage of the 14th amendment, but the wording of the amendment itself placed no such restriction.
Even under more restrictive interpretations that were prevalent in times past, Puerto Rico doesn't qualify. Their gun laws are very similar to other Latin American countries. They require you to have a reason to get a gun, and a "may issue" policy which means they can just say no with no justification required. They don't often say yes, so de facto gun ownership in Puerto Rico is banned, the only people that get them are the politically connected and people who grease palms.
Yeah it is pretty interesting, I read about it a long time ago and having trouble with the search engines, but some kid found a discrepancy between the ND constitution and the US constitution, which was quickly amended and fixed in 2012 (not 1997 as I thought). Basically the ND constitution didn't require the governor to take an oath of allegiance to the US, which the US constitution requires all state constitutions to require officers of the state to do.
Hmm, I wonder how those laws got to be that way. It's a mystery.