Can they take them - Yes
Will they take them - No
Ever hear of New Orleans?
Could they take them - Yes
Did they take them - Yes
Have they returned all of them or compensated the owners - No
NYC requires EVERYTHING to be registered. If they decide to ban all guns within the city you know those weapons will be confiscated.
Gov'ts have a long and proud history of confiscating firearms, why would you think anything is any different now?