Like Anthony says: it's Ringo, so don't try and apply math to it.

(Although I honestly still don't understand why his sunbeam weapon has to have a ceiling on effective damage . . .)

As to the gun in question, Ringo simply ignores how that sort of velocity turns *air* into what is effectively solid matter.

 

IIRC, it's a King-Kong-sized autorifle (I always imagined something like what Warhammer 40K miniatures are shown to carry) that uses antimatter as it's "gunpowder."