We may be a little late to this party, but I just stumbled across some video of a guy firing his homemade .700 WTF (as in “what the F#@& was that!?”) rifle. The cartridge was designed based on a .50 BMG case and using a .700 caliber bullet weighing 1,132 grains (.50 BMG is usually ~600 grains, BTW). It’s not a “destructive device” thanks to a “sporting” exemption, the same one that lets the .950 JDJ keep on this side of the NFA, but is slightly easier to reload for thanks to the use of readily available .50 BMG brass. Probably not something you can put on your holiday wish list just yet, but the videos look like a ton of fun.