In many ways the rifles slug tube can be considered a modern version of the old 'Paradox' gun which was normally a double gun with one or both of the barrels rifled for about two inches at the muzzle.
They were also called 'shot and slug' guns.
Since the rifles tubes are normally cylinder bored or feature a very slight choke boring you may shoot Foster or Brenneke slugs through the tube.
You can shoot shot but the results will be less than satsifying as no choke and rifling will generally produce a donut shaped pattern on target.
The Paradox guns featured a fairly tight choke boring and used a smooth side, hollowbase, undersized, and fairly soft lead slug that expended to fill the bore and then collapsed gradually as it passed through the full length choke boring to safety exit the muzzle spinning and not blowing the muzzle of the gun open as it passed.
These guns also produce a donut shot pattern but because of the choke boring, the pattern is much tighter and the donut hole effect is much smaller on target.
Best results should be found with one of the sabot slug loads available.
If the rather high cost of these cartridges may be a concern I might suggest looking into the Wolf brand slug load which features a modified Brenneke type slug encased in a 3/4 sabot and at very moderate cost, about $3.00 for a box of five on sale.
