Wth ? I thought .50 was the biggest that is publicly available and anything bigger is considered a cannon and not for public use. (never mind that in cal we can't even have the .50) What law is allowing the 20mm to be publicly available ?
I believe that that photo shows a guy holding a Finnish 20 MM semi auto anti tank rifle, WW2 era. When I was a wee lad, one could see them sold via mail for $99.99 each Plus S& H. Ammo was $1.00 each.
Last I checked the 20mm was considered a "demolition device" not technically a firearm and therefore falls under the same regulations as tnt and such.
Anything over .50 caliber is classified as a "Destructive Device" and subject to the same restrictions on purchase as full autos. If you want to pay the tax and do the paper work you can own one. And I belive the rifle pictured is a "Solothurn" anti-tank rifle. You can see plenty of them for sale at Knob Creek every year.