I have converted a few combination guns to double rifles, but sometimes the barrel tapers, shotgun and rifle, are different, right from breech forward, so when you cut barrels to make the monoblock, each side of monoblock is not exactly the same. It varies, of course, from one caliber/gauge to another. However, even on those kinds of combo. guns, if you make a new monoblock from scratch, it works, as you can have both sides of monoblock taper and match in diameters. I believe, to be correct, smaller caliber combination guns are called exactly that, combination guns, while big bore ones are more often referred to as cape guns, as they were used in Africa. On some of these guns, the rifle is proofed for smokeless, while the shotgun barrel is not, and is often Damascus.