
If you check the news today, (AP) the "authorities also found a state-of-the-art
automatic machine gun that investigators did not know was in the cult's arsenal."
[Carl Stern, Justice Department]

I imagine the authorities know the difference between semi and fully automatic
and probably knew weather the guns were legal as they have access to any relative
documentation (i.e. permits).  In addition the .50 caliber guns (plural) were
semi-automatic rifles.