“The officer may have wanted to see if it was fully automatic as well which would have changed its status as to whether it was a legal carry or not.”
The officer may want to do a lot of things, but legally he had no right to confiscate or even inspect the weapon. The cop had the right to conduct a consensual encounter, which is completely voluntary on the part of the subject. In other words, he does not have to stop or talk to the police. Unless the officer could provide articlable facts that rise to the level of reasonable suspicion that a crime has occurred or is about to occur, he had no legal standing to detain or confiscate.
IIRC, cops have the right in most states to disarm you while investigating what is going on. After that, if there is no crime, they have to return the unloaded gun & ammo to you. At least, that is what I was taught in my CCW class in Arizona...