One gun keeps coming to mind with these discussions.   I've usually seen them over at Dixie Gun Works:  Kodiak Double Barreled Rifles, by Pedersoli.  They are black powder, so the reload time is much longer than the current .303 rifle.  However, they are a much higher caliber, delivering an impressive impact upon the target.  Actually, DGW has them in different calibers and combinations.  They have plain double-rifles ranging from .50 cal to .72 cal.  But, more inline with this discussion, they