Please note interchangeable cylinders of .357 Mag (9mm) and
.44 Colt(.45 Auto):
http://www.ruger.com/products/newModelBlackhawkConvertible/models.html
Ruger also makes a Convertible .22 LR/ .22 Mag.
http://www.ruger.com/products/newModelSingleSixConvertible/models.html
That should be .45 Colt not .44.