A babbit bar is bar of rather soft bearing alloy, sort of a hard lead alloy. As for the method, basically you clamp the frame of the revolver in a vice with shaped hardwood inserts and whack the barrel so that the frame is bent slightly at the barrel mount. Naturally this approach did require that the Smith doing the work "have the touch" because otherwise he'd be bending that frame back and forth until it cracked. To be blunt not something for a kitchen table gunsmith and something that still makes me cringe at the thought of. Aren't you glad that today the Lathe is quite common and we can "re clock" a barrel using a technique that allows us to make adjustments down to the 1/10,000 inch.