If you lay your CO2 tank on its side and it is more than half full, liquid state (under pressure) CO2 will run into the regulator. When the liquid reaches the part of the regulator where the pressure is reduced below the boiling point of CO2, the volume of that little bit of liquid CO2 will dramatically increase. I do not believe the regulator is either designed or built to allow for this dramatic increase in volume. Accordingly, you should play it safe and NOT operate a CO2 cylinder on its side.