It is my understanding that anything larger than .50 is considered a NFA destructive device. If this is correct, then aren't most elephant guns considered NFA weapons? like the .600 or .700 Nitro express? not to mention my coveted 4-bore.
As usual, the government is too ill-defined in explaining their make-em-up-as-they-go rules and say that if it is larger bullet diameter than .5 inches it is a destructive device unless it is converted to a signalling device, rescue device, or is an antique or can be demonstrated for sporting purposes only.
Here is one for you...brand new: http://www.centuryarms.com.au/docs/4bore.html Just $35,000.00 smackeroos!
S/N.002. A new Anson & Deeley single barrel rifle chambered for the 4 bore Rewa cartridge, case length 4,1/4", 2000 grn solid lead or copper washed bullet, with extra 4 bore shotgun barrel & its own forend.