The barrel is 6mm, you can get 6mm ammo in metal
It's very expensive though
If you put 4.5 shells & BBs through the 6mm gun they will come out but the velocity will be low as the CO2 will pass the bb in the barrell and grouping wil be pants as they'll rattle about on their way down the loose fitting barrel.
It's entirely possible you could fit a sleeve to reduce the bore to 4.5mm but having one made might cost more than another Dan Wesson,
Regards,
Nick