Anything with a bore larger than .50 caliber is considered a "destructive device" under NFA regulations. Destructive devices aren't illegal, but they have to be registered with the ATF (like SBR's or Suppressors), and certain states have banned their transfer to civilians.
No.
The model gets made with a "sporting exception." If you go buy a .577 double rifle, you don't have to file the ATF paperwork for a DD. Same with a .600 Nitro, etc. Lots of sporting rifle cartridges over .50 and they aren't DD's.
Gregg