Unfortunately, gun barrels are NOT just pieces of threaded pipe that can just be screwed on and off.
If the replacement barrel was a used barrel, it's not at all unusual to find that it can't be used AT ALL.
This is due to several factors.
Often a barrel is removed with improper tooling, and has hidden damage.
A used barrel is often removed because it was damaged, and is unusable.
In any event, ALL revolver barrels MUST be fitted, and the fitting is critical.
Barrel/cylinder gap should be about a minimum of .003, and a max of .010.
The end of the barrel MUST be absolutely SQUARE with the bore, and the only way this can be done is to use a rather expensive cutting tool that is inserted down the bore and uses a special cutting head that screws on the end.
After setting the gap, you then MUST cut or re-cut the forcing cone with a similar expensive tool.
The forcing cone is critical, and it must be gaged with a special measuring device.
The diameter of the OUTER edge of the forcing cone MUST be within specs, or accuracy is poor, and the revolver will spit bullet material.
If the diameter of the cone is too large , OR too small, there will be problems, and the spec is VERY narrow.
Before going any farther, I strongly suggest buying a copy of Jerry Kunhausen's book on gunsmithing the Ruger Revolvers.
This is fairly cheap and can be bought from Brownell's, Midway, and can be ordered from many online and local book stores.
I'm not trying to discourage you, it's just that there is a LOT more to re-barreling a revolver than screwing a barrel on and setting the barrel/cylinder gap.
