There are a few operational uncut tank guns out there. The big dealer has an operational firing m41 on his site right now.
The m5 i am riding in my avatar icon is a firing tank :-)
It takes a destructive device liscense from the BATF to own one of these. There is a Sherman here in San Diego as well this is "Fully Functional" as LtCDR Data would say.

my friend has a cut and welded 106mm recoil less rifle on his jeep that we fire nerf footballs out of. Good fun.