Using a rifled slug in a rifled barrel will do no harm, but I would have to ask why even consider it? I have shot both the smooth bore/rifled slug combo and the rifle barrel/sabot setup. Many times at the range and a few times hunting. The rifled barrel/sabot setup is leaps and bounds of an improvement over a rifled slug/smooth bore. There are numerous articles out there that cover the subject. Some are sort of like this response .... no "real proof". Some are so scientific that they are almost boring. But bottom line is that the smooth bore/rifled slug simply is not as accurate as the sabot/rifled barrel set-up .... and the effective range is much, much less.