oskar,
If Simpson Ltd. doesn't have barrels, I suggest you buy a shotgun from them and use the barrels with the combo setup, when traveling( I think they will fit). When at home, it will still be a "stand alone" gun. These guns are very heavy, but also very robust. Combo barrels were available for full caliber cartridges and are pretty easy to "scope", a combo set in 7x57R would make your "set" very useful.
Mike