"Husquvarna 12s" were made for a VERY long time in several action types, so I think we'd need a pic or much more description to judge this. Just for a start, does it have Damascus barrels? An underlever action? Is the barrel flat marked 65mm or 70mm?

And how tight are the chokes, since you are apparently wanting to use a solid ball or slug? Many of these are 'way too tightly choked for solids, IMO. Or is it possibly rifled?

De debil's in de details!