Serbu Firearms, maker of the Super Shorty 6.5″ barreled shotgun and the BFG-50 and -50A .50 BMG rifles, has published several videos and photos of their current project, the Typhon 20. It is a single shot, bolt action, very similar to the BFG-50, albeit a significant amount larger.
The prototype rifle, as of yet, does not have a stock or a scope mount. It is currently chambered in 20x110mm Hispano, and has a very heavy barrel and no muzzle brake, although Mark Serbu has indicated that production models are planned with lighter barrels and a muzzle brake. He also has said that he intends to make the Typhon in a more common 20mm chambering, such as 20×102 Vulcan, which the Anzio Ironworks 20mm rifle also chambers. It uses a modified version of the trigger pack from the BFG-50.
Test firings were done from a M2 tripod, with a pintle mount attached to the rifle’s handguard. Recoil from the rifle had enough force to pull the pintle out of the tripod, even with the spring loaded detent trying to hold it in place.
As Mark noted on his website’s forum, Serbu Firearms does not have a Destructive Device license yet, and is building this rifle on a BATF Form 1.