Help with Ruger Convertible

[mo shooter]

I purchased two Ruger Blackhawks in .45 Colt. They are a  5 1/2 and a 7 1/2 inch barrel and came with one .45 ACP cylinder, so I assume one of them is a convertible model. Are the convertible cylinders specific to one gun or are they interchangeable? If they are specific, does anyone have any tips on how to tell which gun it goes with?

[Hairtrigger]

The factory will tell you the cylinder needs to be fitted to the gun. For a Old Army you can buy a 45C cylinder (aftermarket) that does not need fitting.
I would call the factory, ask for records dept and give them the S/N of each handgun. They will tell you which revolver the cylinder is for.
Personally It would matter to me

[mcwoodduck]

According to the blue book of gun values there are two models of Convertibles.
The second set is what I think you have.  If not have the guns checked by a good gun smith to tell you what frame the 45 acp cylinder is tuned for.  Very rare to have it tuned for two frames.  From what I under stand.  
The book talks about the new models.  Not the three screw models as being in 4 5/8, 5.5 and 6.5 only with the convertibles, Unless it's stainless and they made them in both 4 5/8 and 7.5 "
The short answer is yes the cylinder is timed to the gun.  So the cylinder hole lines up with the forcing cone.  If you do not mind a little splatter you could shoot the 45 ACP in the 7.5  but I would only use light loads.
Your other option is to contact Ruger and ask them.  Their records will tell you what the gun was shipped with.

[John Traveler]

The Ruger Convertible .45 revolvers I've seen had the serial number etched into the front face of the cylinder. 

If Ruger has stopped this marking practice, you can still identify which frame the convertible cylinder goes to attempting to fit it to both.  With a little luck, it will only fit one frame.  The Ruger factory grinds the cylinder pin bearing surface to fit the frame it is intended for.  The correct frame will give proper barrel-to-cylinder gap and have minimal endshake.

[BlkHawk73]

  Never trust the BB for anything Ruger.  There's simply too many short runs and the like and the BB just doesn't cover them all.  Typically the Blackhawk 45 Colt convertibles are the 4 5/8" and 5 1/2" bbl models.  There have been others though.  Best way, to tell, look at the rear face of the cylinder, there should be three numberes lightly etched into it, each number by a different chamber.  these 3 numbers correspond  to the last 3 digits in the serial number of whichever gun it was shipped with and fitted to.

[mo shooter]

Thanks to all of you for your help. I had already tried checking the fit but the cylinder fits both frames well ... so, no help there. I finally found the three numbers "lightly" engraved on the face of the cylinder. Had to use a magnifying glass to read them but they match the 7 1/2 inch barreled gun's serial number.