There was no 4A violation in the seizure at all. The defendants had probable cause to seize the firearm (and accessories), until such a time that the DA decided not to try and prosecute Al-Mujaahid.
Until that moment, the property was evidence of possible wrong doing. After that moment, his property should have been returned.
The "seizure" I referred to is not the original seizure, it's the continued holding of the property which violates the Fourth Amendment.
For example, a police officer pulls over a motorist for speeding. The officer has no reasonable suspicion or probable cause to believe the motorist has drugs but decides to play a hunch. The officer radios for Drug Dog Doug to come and do his thing while the stopping officer verifies insurance, checks for warrants, etc. The motorist has been seized, but it's lawful up until the time all of the queries about warrants, etc. come back empty.
If Doug gets there by then, he can walk around the car and sniff. If he alerts, this may give probable cause to search the car. If the officer continues holding the motorist waiting for Doug beyond the time necessary to conduct the usual checks, the seizure of the motorist becomes unlawful and any "hit" by Doug and resulting search are tossed.
That's essentially what has happened here. The initial seizure may have been lawful but once the reason for the seizure ends, the continued possession of it violates the 4th Amendment prohibition against unreasonable seizures.