My good friend John Foster ,who many of you know of for high quality sleeving ,just refers to the cut off barrels as the " back ends".A term that is used extensively in the UK trade that fits the bill. It may be of historical note that the man accredited with "inventing" sleeving in the UK Chris Ashthorpe disgusted at the price of a refit, decided to sleeve a pair of barrels as he had a Belgian gun built on a monoblock he could see no reason why it could not be done to an existing pair of barrels. He then had a considerable fight with the proof authorities, to get them to accept it. In the early days, some of the Birmingham trade referred to them as being trouser legged.