the .577 and .700 are illegal in the united states a .50 bmg is barley legal
Lots of the large bore rifles get a Destructive Device exemption from the BATFE because they are intended for sporting purposes. Think of all the over 50caliber British rifles you can get. Then consider some of the JDJ cartridges. I could have SSK build me a 700 JDJ rifle and all it takes is $$$$.
For the some of the bigger stuff like the 620 JDJ handgun, all that you need is the BATFE $200 federal tax stamp for a destructive device, similar to owning full auto weapons where it's legal.
This particular caliber seems very similar to the 12 ga FH which is just a brass cased 12ga used in rifled barrels and loaded as a rifle instead of a shotgun. The 12ga FH uses modified BMG brass as well.
Andrew