They either have permits, or are criminals. I know one person with tanks- he has permits, they are accounted for, and he runs a museum park- which he pays for, and is required to open to the public at times) to keep his park standing.

Large caliber ( not sure of lower limit, but 20MM is included) are restricted as destructive devices. There are a number of 3" guns at AL halls, but ALL have been de-militerized and made incapable of use ( weilded recievers, filled barrals, etc.

I can get a machine gun- de-militerized- that means it has been rended incapable of being meade operable.