I've got one as well, it is classified as AOW and I would guess that is due to the O.A.L. being below the minimum length which I think is 26". No big deal to get though, just a $5 stamp.
Side question on this issue:
A friend has a single shot shotgun that had the barrel damaged and in order to cut it and recrown it, would put it just under 18". Replacement barrels are not produced and he has searched the net and gun shows, but to no avail.
Now, on to the rat killin', could he apply for an AOW stamp and then have the barrel cut down?
I've looked into some things, but had not thought of the AOW category.....thanks.