It takes special equipment and expertise - usually at the factory level, or by a gunsmith with such training - to change a revolver barrel. It is screwed extremely tightly into the frame, and your new barrel may not index correctly when screwed back into the frame, usually requiring machining of the shoulder, an equal amount removed from the breech end and the forcing cone recut. Ruger will not do it as your donor barrel was not originally made for your particular gun, and it might well exceed the cost of another gun with the desired barrel length to have a gunsmith do it.