I think that there is some confusion over terminology here. Sleeving refers to cutting off the original shotgun barrels, just forward of the flats, and screwing, or soldering, or both, the rifle barrels into the monoblock thus created. The barrels can thus be lathe-turned to be as heavy or as light as desired. In the insert process, the shotgun barrels are bored out, full length, and the rifle barrels are turned to a diameter that will allow them to fit inside the shotgun barrels. This, of necessity, results in a very heavy, weight-forward, and awkward rifle.