The regulations change from state-to-state.
Here in NY (St Lawrence County), I've checked with the county clerk (that's where we get our pistol permits) and she stated the following.
If you want an Encore/Contender pistol and you already have a rifle or muzzle loader, go ahead and get a pistol barrel if you want to, but come in with the serial number of the frame and the caliber of the pistol barrel and register it before you put it together.
In some states, you must buy the frame as a pistol or you will violate the law by putting a pistol barrel on the frame.