Using ab initio total energy calculations we demonstrate that the recently found metal encapsulated silicon clusters Si12Be can be assembled to form hexagonal nanotubes of silicon. This is in contrast to undoped silicon structures that are distorted and have a tendency to agglomerate. The finite nanotubes have varying HOMO-LUMO gaps depending upon the length and amount of doping. However, infinite nanotubes are metallic, symmetric and stable, making metal encapsulation a useful route to generate metallic silicon nanowires for miniature devices.