Howdy
The ONLY newly manufactured Single Action cartridge revolvers legal for sale in Mass are Rugers. NO newly manufactured cartridge single action revolvers with a traditional colt type lockwork are legal for sale in Mass. This includes Colts and ALL their clones. The only loophole in this situation is if you can find a revolver that was manufactured before October 1998 AND it was registered in the state prior to that date. Unfortunately, guns manufactured previous to that date that were not registered in the state are NOT legal for sale here. You can occaisionally find a pre 1998 'Mass Compliant' gun that is legal here, I have found some, but you have to hunt for them.
Here is a link to the Department of Public Safety's list of handguns approved for sale in Mass. Take note at the beginning to the disclaimer about guns being on the list that may not meet with the Attorney General's safety specifications, for instance the S&W Schofields manufactured a few years ago. They are on the list, but they will not pass a drop test. I don't know of anyone selling them, even if there were any left.
http://www.mass.gov/Eeops/docs/chsb/firearms/approved_firearms_roster_10_2007.pdfPercussion revolvers are a little bit iffy. Technically, under Federal law they are not firearms. Some dealers will sell them, some are not sure so they don't touch them. The real fly in the ointment is that the Ruger Old Army is not Mass legal.
The only other loophole is a Federally issued Curio and Relic license. This will allow the holder to buy guns on the C&R list that are not normally legal for sale in Mass. For instance, ALL 2nd Gen Colts are on the C&R list. Generally speaking, any gun that is at least 50 years old is automatically considered C&R eligable. In addition to the C&R license you will need a normal Mass issued License to Carry in order to legally own any handguns you purchase on a C&R. I have purchased several old guns on my C&R that I would not have been able to purchase on my Mass License to Carry. I like old guns.