Joining barrels can be simple or it can be complicated. It just depends on how you go about it. Planning and preparation is the key to a less complicated attempt. As for shims, I have seen barrel sets with them, without them and packed with clay in between. As for the barrels, first I have to ask if they are chambered breech loading barrels already tapered or are they simple straight tubes?

Yes, I did write a book. It's called "The Classic English Double Barrel"