 Rarer were the drillings that used two rifled barrels and a single smoothbore barrel.These were harder to make, since, like a double rifle, the rifled barrels must be very carefully "regulated", that is, aligned during manufacture to shoot to the same point of aim at a given distance.This requires more precision than regulation of double-barreled shotgun barrels, which are used at shorter ranges with wide patterns of shot where a small misalignment won't be significant.