This is the one I'm most familiar with.
https://www.gunsitters.com Locations in PA, MD, and NJ. I know a lot of shooters in NJ who keep guns and/or magazines no longer legal in NJ at the PA location, as there are a number of gun clubs near that site where they can shoot them. Usually because they are either hoping some day the laws will change back and allow the guns to be brought back home, or because the irreversible alterations required to make some collectibles NJ-legal would destroy their value.
The NJ site is popular because threatening to file a domestic violence complaint in order to obtain concessions in property division is rapidly becoming SOP in ugly divorce cases. There are no repercussions for filing a false complaint here, and the guns get confiscated. Lawyers increasingly recommend getting firearms to legal offsite storage if a divorce case starts to get ugly.