Howdy,
Nope, it is nominal .535 to .540. The Rapine mold throws a 375 grain, .535 heeled bullet. The case mouth crimps onto the rebated heel (about .515 dia) of the bullet. In the Civil War, they sometimes refered to them as .52 caliber Spencers, though this was because the bore (from land to land) was about .52. The grooves are about .540, if I remember what mine slugged at.