Used for connecting two pipes of different sizes. This is different then a reducer in the respects that a reducer connects two male pieces, meanwhile a bushing connects both a female and male pipe. The larger side is for inserting into female connections and the smaller side connects to male connections.
Often used to connected the barrel to the chamber and doesn't angle on the outside but rather the inside. Most commonly the bushing is inserted into a coupling, but will connect to any female connection that is both the same size and connection type (Slip or Thread).
Some people modify them so that the stop on the inside of it is no longer present.