can a 44-40 be rechambered to a .44 mag
Yes, like Sitka asked, "what gun"?
Back in the 1950's and 60's, is was common for 'smiths to rechamber Winchester 1892s from 44-40 to 44 Mag, but I have been told that there were feeding issue to overcome in that conversion.
If the firearm is a revolver, swapping cylinders would seem like an easier solution.