Sleeve the the Model 57's cylinder throats for a tight fit with .401" bullets. From the other end ream the chambers to about .490" forward to the start of the throats. Face off the back end of the cylinder for clearance to weld on a washer that covers the chambers. The washer has to be thick enough to drill and thread for muzzle loader percussion cap nipples. Cast your 200 grain 10mm bullets out of pure lead and a lightly compressed charge of FFFg black powder will bump the bullet up to fill the grooves.

I leave it to Alk8944 to work out the details.