The theory of filing the muzzle out of square is the escaping gas will push the load opposite the low side. In an old Dixie Gunworks, black powder annual was an article where a gentleman had a collection of St Louis made SxS doubles meant for shooting ball. Several of them had the muzzles out of square in this manner. As I recall at this later date they were made by Hawkins or Meecham.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra