I'm not a lawyer.
But, I don't see a rights violation from the viewpoint of the law.
News reports are always suspect as to full facts and no misrepresentation, intentional or otherwise. But, lets approach this on the assumption that the news report is accurate and complete, knowing it still might be wrong.
Facts:
The husband and wife consensually answered questions.
They opened their door, or more importantly did not close it when cops showed up.
The police asked the husband to hold out his hands and he consented.
The police found an old spent shell casing on the back porch. Without a report from the homeowner that he refused consent to the police going back there without a warrant, lets assume the husband consented to a request to "look around."
Analysis:
The police arrived for a knock-and-talk (consensual encounter). The homeowners, by answering questions and letting cops look around, consensually may have given the police probable cause (shell casing, ballistic evidence on hands). If that "probable cause" is supported by the courts, then seizing the guns might well be legal since they are instruments of an offense and evidence.
As far as the police not checking the gun barrels for temperature, smell, or residue, I'm not so sure they are required to check. I've read cases where police were not required to exclude innocent explanations before moving forward with their suspicions--this in terms of reasonable suspicion during a Terry detention. If some version of that rationale carries over into probable cause, the gun seizures may well be judged legal by a court.
Observation:
Don't talk to police. Don't talk to police. Don't talk to police!!
The cops violated their rights. Maybe not those few remaining rights the gunverment is reluctantly willing to recognize, but the rights they deserve. How the heck can you see ballistic residue on someone's hands, unless he was shooting a revolver, and more than a few rounds? A shell casing on the back porch? Really? A shell casing when shots were reported?
And, then not actually checking to see if any of the guns were fired very recently? Come on. Anybody with half a lick of experience with a gun can tell the difference between the sharp, distinct odor of a freshly fired barrel, and one that was shot yesterday or earlier. If the cops had one lick of decency, they would have checked. These cops were anti-gun or looking to add to their job stats.
I do find a couple problems in the report.
First, the police said they did not harass or intimidate the couple. Yet, the couple clearly report the police threatening to arrest the husband. Huh!?! Why would a cop threaten to arrest someone? The cop either has probable cause for an arrest or he doesn't. If a threat is made, it is clearly intended to intimidate.
Lastly, the report puts the threat to arrest right in front of the request to hold out hands. If the husband was threatened with arrest and the request to hold out hands was immediately after that, then I would say the hand gesture was not consensual, but the product of fear. I'm not so sure the police can legally do that without probable cause. I'm thinking such an inspection would be a search. Think about it for a moment. If the so-called residue supplied the final point of probable cause for an arrest or gun seizure, and the cops gained that info by intimidation, such would be manifestly unfair.